[JavaScript] (PR provided) Cruft causing incorrect highlighting for standard JavaScript: Mustache templating, HTML comments, IE conditional compilation

[Thom1729]

The following tests fail:

{{ 42; }}
// ^^ meta.block.js meta.block.js constant.numeric.js

x --> y;
//^^ keyword.operator.arithmetic.js
//  ^ keyword.operator.relational.js

/*@if /*/
//     ^^ punctuation.definition.comment.js

z
// <- variable.other.readwrite.js

Each test is valid JavaScript, but the parsing is broken by code that handles non-JavaScript syntax:

• Highlighting for mustache templates conflicts with JavaScript blocks. This is fundamentally unfixable; the two languages conflict. If we want to maintain mustache embedding in JavaScript code for some reason, we can make a new syntax for it (simply including JavaScript with a with_prototype). Or, if it's not worth having a separate syntax for, it's probably not worth breaking the JavaScript syntax for.
• Highlighting for HTML comments conflicts with JavaScript operators. This could conceivably be patched, because <!-- cannot conflict with real JavaScript at top level. But the implementation seems rather pointless to begin with, because it can't cover all uses of HTML comments (e.g. in strings) without breaking JavaScript and the HTML syntax should handle this anyway using with_prototype.
• Highlighting for ancient IE conditional compilation breaks JavaScript comments. This is an ill-advised and archaic practice that has long been abandoned by Microsoft, so it should be safe to just cut it.

Fixing these will help to clear the way for #885 and related issues.

[Thom1729]

I'm closing this. The three bugs described here are addressed in more detail elsewhere.

Mustache templates: #927
HTML comments: #928
IE conditional compilation: TBD