enh(parser) multi-class in a single mode

[joshgoebel]

Resolves #2838.

Changes

Adds multi-class highlighting:

{
        begin: [
          /function|function!/,
          /\s+/,
          hljs.IDENT_RE
        ],
        className: {
          1: "keyword",
          3: "title"
        },
}

Checklist

• Added markup tests, or they don't apply here because...
• needs unit tests
• [x ] docs
• Updated the changelog at CHANGES.md

[allejo]

Loving the simplicity needed to add this feature

[allejo]

This seems pretty sane to me

Edit: Waittttttt! See #3081 (review)

[allejo]

Suggested change

	isMultiClass?: boolean,
	isMultiClass?: true,

If there's no time it'll ever false, should the type be true instead of boolean?

[joshgoebel]

Perhaps technically, but that bugs me though i can't explain it. This isn't the same as isCompiled IMHO.