Expose context server settings to extensions (#20555)

This PR exposes context server settings to extensions.

Extensions can use `ContextServerSettings::for_project` to get the
context server settings for the current project.

The `experimental.context_servers` setting has been removed and replaced
with the `context_servers` setting (which is now an object instead of an
array).

Release Notes:

- N/A

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/extension_host/src/wasm_host.rs

@@ -27,7 +27,7 @@ use wasmtime::{
 };
 use wasmtime_wasi as wasi;
 use wit::Extension;
-pub use wit::SlashCommand;
+pub use wit::{ExtensionProject, SlashCommand};
 
 pub struct WasmHost {
     engine: Engine,