Rewrite the Molang Autocomplete system to improve plugin extensibility.

[SnaveSutit]

This PR is a major rewrite of the Molang Autocomplete system in an attempt to allow plugins to modify it without overwriting it.

There are extensive comments throughout the code, and partial JSdoc typing.

I'm opening this PR now to allow Jannis and others to take a peek at it, and make suggestions.

If/when this PR is merged, I'll update the TS types to reflect these changes.