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/lsp_ext.rs

@@ -0,0 +1,54 @@
+use std::sync::Arc;
+
+use crate::Editor;
+use gpui::{Model, WindowContext};
+use language::Buffer;
+use language::Language;
+use lsp::LanguageServerId;
+use multi_buffer::Anchor;
+
+pub(crate) fn find_specific_language_server_in_selection<F>(
+    editor: &Editor,
+    cx: &WindowContext,
+    filter_language: F,
+    language_server_name: &str,
+) -> Option<(Anchor, Arc<Language>, LanguageServerId, Model<Buffer>)>
+where
+    F: Fn(&Language) -> bool,
+{
+    let Some(project) = &editor.project else {
+        return None;
+    };
+    let multibuffer = editor.buffer().read(cx);
+    editor
+        .selections
+        .disjoint_anchors()
+        .into_iter()
+        .filter(|selection| selection.start == selection.end)
+        .filter_map(|selection| Some((selection.start.buffer_id?, selection.start)))
+        .filter_map(|(buffer_id, trigger_anchor)| {
+            let buffer = multibuffer.buffer(buffer_id)?;
+            let language = buffer.read(cx).language_at(trigger_anchor.text_anchor)?;
+            if !filter_language(&language) {
+                return None;
+            }
+            Some((trigger_anchor, language, buffer))
+        })
+        .find_map(|(trigger_anchor, language, buffer)| {
+            project
+                .read(cx)
+                .language_servers_for_buffer(buffer.read(cx), cx)
+                .find_map(|(adapter, server)| {
+                    if adapter.name.0.as_ref() == language_server_name {
+                        Some((
+                            trigger_anchor,
+                            Arc::clone(&language),
+                            server.server_id(),
+                            buffer.clone(),
+                        ))
+                    } else {
+                        None
+                    }
+                })
+        })
+}