lsp: Add support for clangd's inactiveRegions extension (#26146)

Closes #13089 

Here we use `experimental` to advertise our support for
`inactiveRegions`. Note that clangd does not currently have a stable
release that reads the `experimental` object (PR
https://github.com/llvm/llvm-project/pull/116531), this can be tested
with one of clangd's recent "unstable snapshots" in their
[releases](https://github.com/clangd/clangd/releases).

Release Notes:

- Added support for clangd's `inactiveRegions` extension.

![Screen Recording 2025-03-05 at 22 39
58](https://github.com/user-attachments/assets/ceade8bd-4d8e-43c3-9520-ad44efa50d2f)

---------

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

crates/editor/src/clangd_ext.rs

@@ -7,7 +7,7 @@ use crate::lsp_ext::find_specific_language_server_in_selection;
 
 use crate::{element::register_action, Editor, SwitchSourceHeader};
 
-const CLANGD_SERVER_NAME: &str = "clangd";
+use project::lsp_store::clangd_ext::CLANGD_SERVER_NAME;
 
 fn is_c_language(language: &Language) -> bool {
     return language.name() == "C++".into() || language.name() == "C".into();
@@ -46,7 +46,7 @@ pub fn switch_source_header(
         project.request_lsp(
             buffer,
             project::LanguageServerToQuery::Other(server_to_query),
-            project::lsp_ext_command::SwitchSourceHeader,
+            project::lsp_store::lsp_ext_command::SwitchSourceHeader,
             cx,
         )
     });