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/lsp/src/lsp.rs

@@ -299,34 +299,6 @@ pub struct AdapterServerCapabilities {
     pub code_action_kinds: Option<Vec<CodeActionKind>>,
 }
 
-/// Experimental: Informs the end user about the state of the server
-///
-/// [Rust Analyzer Specification](https://github.com/rust-lang/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#server-status)
-#[derive(Debug)]
-pub enum ServerStatus {}
-
-/// Other(String) variant to handle unknown values due to this still being experimental
-#[derive(Debug, PartialEq, Deserialize, Serialize, Clone)]
-#[serde(rename_all = "camelCase")]
-pub enum ServerHealthStatus {
-    Ok,
-    Warning,
-    Error,
-    Other(String),
-}
-
-#[derive(Debug, PartialEq, Deserialize, Serialize, Clone)]
-#[serde(rename_all = "camelCase")]
-pub struct ServerStatusParams {
-    pub health: ServerHealthStatus,
-    pub message: Option<String>,
-}
-
-impl lsp_types::notification::Notification for ServerStatus {
-    type Params = ServerStatusParams;
-    const METHOD: &'static str = "experimental/serverStatus";
-}
-
 impl LanguageServer {
     /// Starts a language server process.
     #[allow(clippy::too_many_arguments)]