Add getLookup() API to allow composing custom lookup from internal lookup

Proposing to add a getLookup API. For my use case, I have some proprietary paths in a stylesheet that are not handled by existing lookups. While I can register a custom lookup to resolve these proprietary paths, I only need to handle certain of those paths, and want to defer the rest to existing lookups. Currently, the tsLookup and some others aren't exported - rather than exporting those, I thought this solution could be more flexible and future-proof if new internal lookups are added.

If this looks desirable, I can do another pass to update docs. Thanks!

Please make sure tests pass, CI didn't run here for some reason.

@rmunch please update the docs to mention the change, thanks.