theme_importer: Improve syntax token matching (#3549)

This PR improves the approach we use to match syntax tokens between Zed
and VS Code in the `theme_importer`.

We now use the list of scopes assigned to each Zed syntax token to rank
the possible candidates in the VS Code and then pick the candidate with
the highest rank.

So far this has proved to provide better colors across the board, but
we'll continue to refine the matching over time.

Release Notes:

- N/A