Refactor LSP adapter methods to compute labels in batches (#10097)

Once we enable extensions to customize the labels of completions and
symbols, this new structure will allow this to be done with a single
WASM call, instead of one WASM call per completion / symbol.

Release Notes:

- N/A

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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>