`DidChangeTextDocument` notification is slow to process and may block

We handle client to server notifications synchronously, which means we should make sure that these notifications are processed fast as otherwise the server will block on them. If a user opens a very large rust file (100k+ lines) they will start encountering overly long loop and the general editor experience becomes very laggy. We can't make the processing for this notification async unfortunately so we should try and optimize the things we do there instead.

Relevant code snippets:

https://github.com/rust-lang/rust-analyzer/blob/bbcb77ea6fc08df598b20267afd6a44018b21f5b/crates/rust-analyzer/src/main_loop.rs#L746-L768
https://github.com/rust-lang/rust-analyzer/blob/bbcb77ea6fc08df598b20267afd6a44018b21f5b/crates/rust-analyzer/src/lsp_utils.rs#L135-L183
https://github.com/rust-lang/rust-analyzer/blob/bbcb77ea6fc08df598b20267afd6a44018b21f5b/crates/ide-db/src/line_index.rs#L57-L94

The main part here is the LineIndex calculation as we have to traverse the entire document text character by character, so we should look into possibly optimizing the creation of it.

Read the notification handling in crates/rust-analyzer/src/main_loop.rs and lsp_utils.rs, then inspect LineIndex creation in crates/ide-db/src/line_index.rs. Profile processing for a 100k+ line Rust file, focusing on the character-by-character LineIndex calculation. Done means the notification completes faster without changing its behavior or blocking the server as severely.