clangd: Implement switch source/header extension (#14646)

Release Notes:

- Added switch source/header action for clangd language server (fixes
[#12801](https://github.com/zed-industries/zed/issues/12801)).

Note: I'm new to both rust and this codebase. I started my
implementation by copying how rust analyzer's "expand macro" LSP
extension is implemented. I don't yet understand some of the code I
copied (mostly the way to get the `server_to_query` in `clangd_ext.rs`
and the whole proto implementation).

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/editor/src/editor.rs

@@ -15,6 +15,7 @@
 pub mod actions;
 mod blame_entry_tooltip;
 mod blink_manager;
+mod clangd_ext;
 mod debounced_delay;
 pub mod display_map;
 mod editor_settings;
@@ -30,6 +31,7 @@ mod inlay_hint_cache;
 mod inline_completion_provider;
 pub mod items;
 mod linked_editing_ranges;
+mod lsp_ext;
 mod mouse_context_menu;
 pub mod movement;
 mod persistence;