This PR adds the ability for extensions to implement
`label_for_completion` to customize completions coming back from the
language server.

We've used the Gleam extension as a motivating example, adding
`label_for_completion` support to it.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>