Add support for context server extensions (#20250)

This PR adds support for context servers provided by extensions.

To provide a context server from an extension, you need to list the
context servers in your `extension.toml`:

```toml
[context_servers.my-context-server]
```

And then implement the `context_server_command` method to return the
command that will be used to start the context server:

```rs
use zed_extension_api::{self as zed, Command, ContextServerId, Result};

struct ExampleContextServerExtension;

impl zed::Extension for ExampleContextServerExtension {
    fn new() -> Self {
        ExampleContextServerExtension
    }

    fn context_server_command(&mut self, _context_server_id: &ContextServerId) -> Result<Command> {
        Ok(Command {
            command: "node".to_string(),
            args: vec!["/path/to/example-context-server/index.js".to_string()],
            env: Vec::new(),
        })
    }
}

zed::register_extension!(ExampleContextServerExtension);
```

Release Notes:

- N/A

crates/extension_api/src/extension_api.rs

@@ -129,6 +129,11 @@ pub trait Extension: Send + Sync {
         Err("`run_slash_command` not implemented".to_string())
     }
 
+    /// Returns the command used to start a context server.
+    fn context_server_command(&mut self, _context_server_id: &ContextServerId) -> Result<Command> {
+        Err("`context_server_command` not implemented".to_string())
+    }
+
     /// Returns a list of package names as suggestions to be included in the
     /// search results of the `/docs` slash command.
     ///
@@ -270,6 +275,11 @@ impl wit::Guest for Component {
         extension().run_slash_command(command, args, worktree)
     }
 
+    fn context_server_command(context_server_id: String) -> Result<wit::Command> {
+        let context_server_id = ContextServerId(context_server_id);
+        extension().context_server_command(&context_server_id)
+    }
+
     fn suggest_docs_packages(provider: String) -> Result<Vec<String>, String> {
         extension().suggest_docs_packages(provider)
     }
@@ -299,6 +309,22 @@ impl fmt::Display for LanguageServerId {
     }
 }
 
+/// The ID of a context server.
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone)]
+pub struct ContextServerId(String);
+
+impl AsRef<str> for ContextServerId {
+    fn as_ref(&self) -> &str {
+        &self.0
+    }
+}
+
+impl fmt::Display for ContextServerId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
 impl CodeLabelSpan {
     /// Returns a [`CodeLabelSpan::CodeRange`].
     pub fn code_range(range: impl Into<wit::Range>) -> Self {

crates/extension_api/wit/since_v0.2.0/extension.wit

@@ -135,6 +135,9 @@ world extension {
     /// Returns the output from running the provided slash command.
     export run-slash-command: func(command: slash-command, args: list<string>, worktree: option<borrow<worktree>>) -> result<slash-command-output, string>;
 
+    /// Returns the command used to start up a context server.
+    export context-server-command: func(context-server-id: string) -> result<command, string>;
+
     /// Returns a list of packages as suggestions to be included in the `/docs`
     /// search results.
     ///