Keep go-to-def responsive even if there's no AST yet (use heuristics in that case)

[HighCommander4]

If go-to-definition is invoked on a file that doesn't have an AST built yet, we currently block on building the AST before sending the response.

As building the AST (preamble) can take multiple seconds (or worse) for large files, this leads to a not-very-responsive experience if you're trying to use go-to-def shortly after opening a file.

Rather than blocking, it might be better to give a heuristic response immediately. We have an existing textual fallback for go-to-def (locateSymbolTextually) which uses index lookup of the word under the cursor. We can create a variant of this that doesn't require an AST at all, and use it.