fix: use Plurimath as asset

Plurimath opal cannot be minified, esbuilded and bundled without causing unexpected exceptions
So copy js in public folder and import plurimath from here

public/plurimath/index.js

@@ -0,0 +1,25 @@
+import "./plurimath-opal.js";
+Opal.require("plurimath");
+window.Plurimath = class Plurimath {
+    constructor(data, format) {
+        this.data = Opal.Plurimath.Math.$parse(data, format);
+    }
+    toAsciimath() {
+        return this.data.$to_asciimath();
+    }
+    toLatex() {
+        return this.data.$to_latex();
+    }
+    toMathml() {
+        return this.data.$to_mathml();
+    }
+    toHtml() {
+        return this.data.$to_html();
+    }
+    toOmml() {
+        return this.data.$to_omml();
+    }
+    toDisplay(lang) {
+        return this.data.$to_display(lang);
+    }
+}