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

---------

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

crates/multi_buffer/src/multi_buffer.rs

@@ -7,7 +7,6 @@ use collections::{BTreeMap, Bound, HashMap, HashSet};
 use futures::{channel::mpsc, SinkExt};
 use git::diff::DiffHunk;
 use gpui::{AppContext, EventEmitter, Model, ModelContext};
-pub use language::Completion;
 use language::{
     char_kind,
     language_settings::{language_settings, LanguageSettings},