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>