Add a super simple wrapper for a merged string table. (#119488)

Suggestions welcome on what to better name this -- `StringTable` as I
currently have it seems too general, but wasn't sure what other

Add a super simple wrapper for a merged string table. (#119488)

Suggestions welcome on what to better name this -- `StringTable` as I
currently have it seems too general, but wasn't sure what other name
would be better.

It currently has a *very* minimal API. I'm happy to expand it if folks
have ideas for what API would be useful, but this actually seemed like
it might be all we really need.

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