refactor(JS): Encapsulated JS in a module instead of using the gobal scope

[JBBianchi]

This refactor encapsulate the JS code in its own module instead of using the global scope which could be tampered with.

Important notice, support for dotnet standard 2.0 has been dropped because of the use of IJSObjectReference.

The JS has been cleaned a bit:

• Removed the use of nested objects and (possibly) misleading this
• Used arrow functions everywhere instead of some kind of mix
• Removed deprecated var, enforced proper use of const/let (it was used randomly)
• Added missing null checking (of editor for instance)
• Used (de)structuring/spread operator

- /*...*/.editors.push({ id: id, editor: editor, dotnetRef: dotnetRef });
+ editors.push({ id, editor, dotnetRef });

- editorHolder.editor.addAction({
-     id: actionDescriptor.id,
-     label: actionDescriptor.label,
-     keybindings: actionDescriptor.keybindings,
-     precondition: actionDescriptor.precondition,
-     keybindingContext: actionDescriptor.keybindingContext,
-     contextMenuGroupId: actionDescriptor.contextMenuGroupId,
-     contextMenuOrder: actionDescriptor.contextMenuOrder,
-     run: function (editor, args) {
-         editorHolder.dotnetRef.invokeMethodAsync("ActionCallback", actionDescriptor.id);
-     }
- })
+ editorHolder.editor.addAction({
+     ...actionDescriptor,
+     run: (editor, args) => editorHolder.dotnetRef.invokeMethodAsync("ActionCallback", actionDescriptor.id)
+ });

• Replaced null checking with shorthands:

- if (model == null)
+ if (!model)

- if (options == null)
-     options = {};
+ options = options || {};

- return editorHolder == null ? null : editorHolder.editor;
+ return editorHolder?.editor;

• Enforced string comparisons (===/!== instead of ==/!=).
• Avoided unnecessary rewraps

- defineTheme: function (themeName, themeData) {
-     monaco.editor.defineTheme(themeName, themeData);
-  }
+ export const defineTheme = monaco.editor.defineTheme;

[serdarciplak]

Thanks for this PR. As this is a breaking change, I'll merge these changes in v4.0.0