restructure: move bin/ and tests/ to src/

Also add symlinks from the old `bin/` and `tests/` locations to avoid
breaking scripts and other tools.

Motivations:

  * Scripts and tests no longer have to do dubious things like:

        require('ep_etherpad-lite/node_modules/foo')

    to access packages installed as dependencies in
    `src/package.json`.

  * Plugins can access the backend test helper library in a non-hacky
    way:

        require('ep_etherpad-lite/tests/backend/common')

  * We can delete the top-level `package.json` without breaking our
    ability to lint the files in `bin/` and `tests/`.

    Deleting the top-level `package.json` has downsides: It will cause
    `npm` to print warnings whenever plugins are installed, npm will
    no longer be able to enforce a plugin's peer dependency on
    ep_etherpad-lite, and npm will keep deleting the
    `node_modules/ep_etherpad-lite` symlink that points to `../src`.

    But there are significant upsides to deleting the top-level
    `package.json`: It will drastically speed up plugin installation
    because `npm` doesn't have to recursively walk the dependencies in
    `src/package.json`. Also, deleting the top-level `package.json`
    avoids npm's horrible dependency hoisting behavior (where it moves
    stuff from `src/node_modules/` to the top-level `node_modules/`
    directory). Dependency hoisting causes numerous mysterious
    problems such as silent failures in `npm outdated` and `npm
    update`. Dependency hoisting also breaks plugins that do:

        require('ep_etherpad-lite/node_modules/foo')

bin/plugins/lib/npmpublish.yml

@@ -1,83 +0,0 @@
-# This workflow will run tests using node and then publish a package to the npm registry when a release is created
-# For more information see: https://help.github.com/actions/language-and-framework-guides/publishing-nodejs-packages
-
-name: Node.js Package
-
-on:
-  pull_request:
-  push:
-    branches:
-      - main
-      - master
-
-jobs:
-  test:
-    runs-on: ubuntu-latest
-    steps:
-      # Clone ether/etherpad-lite to ../etherpad-lite so that ep_etherpad-lite
-      # can be "installed" in this plugin's node_modules. The checkout v2 action
-      # doesn't support cloning outside of $GITHUB_WORKSPACE (see
-      # https://github.com/actions/checkout/issues/197), so the repo is first
-      # cloned to etherpad-lite then moved to ../etherpad-lite. To avoid
-      # conflicts with this plugin's clone, etherpad-lite must be cloned and
-      # moved out before this plugin's repo is cloned to $GITHUB_WORKSPACE.
-      - uses: actions/checkout@v2
-        with:
-          repository: ether/etherpad-lite
-          path: etherpad-lite
-      - run: mv etherpad-lite ..
-      # etherpad-lite has been moved outside of $GITHUB_WORKSPACE, so it is now
-      # safe to clone this plugin's repo to $GITHUB_WORKSPACE.
-      - uses: actions/checkout@v2
-      - uses: actions/setup-node@v1
-        with:
-          node-version: 12
-      # All of ep_etherpad-lite's devDependencies are installed because the
-      # plugin might do `require('ep_etherpad-lite/node_modules/${devDep}')`.
-      # Eventually it would be nice to create an ESLint plugin that prohibits
-      # Etherpad plugins from piggybacking off of ep_etherpad-lite's
-      # devDependencies. If we had that, we could change this line to only
-      # install production dependencies.
-      - run: cd ../etherpad-lite/src && npm ci
-      - run: npm ci
-      # This runs some sanity checks and creates a symlink at
-      # node_modules/ep_etherpad-lite that points to ../../etherpad-lite/src.
-      # This step must be done after `npm ci` installs the plugin's dependencies
-      # because npm "helpfully" cleans up such symlinks. :( Installing
-      # ep_etherpad-lite in the plugin's node_modules prevents lint errors and
-      # unit test failures if the plugin does `require('ep_etherpad-lite/foo')`.
-      - run: npm install --no-save ep_etherpad-lite@file:../etherpad-lite/src
-      - run: npm test
-      - run: npm run lint
-
-  publish-npm:
-    if: github.event_name == 'push'
-    needs: test
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-node@v1
-        with:
-          node-version: 12
-          registry-url: https://registry.npmjs.org/
-      - run: git config user.name 'github-actions[bot]'
-      - run: git config user.email '41898282+github-actions[bot]@users.noreply.github.com'
-      - run: npm ci
-      - run: npm version patch
-      - run: git push --follow-tags
-      # `npm publish` must come after `git push` otherwise there is a race
-      # condition: If two PRs are merged back-to-back then master/main will be
-      # updated with the commits from the second PR before the first PR's
-      # workflow has a chance to push the commit generated by `npm version
-      # patch`. This causes the first PR's `git push` step to fail after the
-      # package has already been published, which in turn will cause all future
-      # workflow runs to fail because they will all attempt to use the same
-      # already-used version number. By running `npm publish` after `git push`,
-      # back-to-back merges will cause the first merge's workflow to fail but
-      # the second's will succeed.
-      - run: npm publish
-        env:
-          NODE_AUTH_TOKEN: ${{secrets.NPM_TOKEN}}
-
-##ETHERPAD_NPM_V=2
-## NPM configuration automatically created using bin/plugins/updateAllPluginsScript.sh