extension: Reorganize capabilities (#35143)

This PR reorganizes the capabilities within the `extension` crate to
make it easier to add more.

Release Notes:

- N/A

crates/extension/src/extension.rs

@@ -1,3 +1,4 @@
+mod capabilities;
 pub mod extension_builder;
 mod extension_events;
 mod extension_host_proxy;
@@ -16,6 +17,7 @@ use language::LanguageName;
 use semantic_version::SemanticVersion;
 use task::{SpawnInTerminal, ZedDebugConfig};
 
+pub use crate::capabilities::*;
 pub use crate::extension_events::*;
 pub use crate::extension_host_proxy::*;
 pub use crate::extension_manifest::*;