Add initial support for defining language server adapters in WebAssembly-based extensions (#8645)

This PR adds **internal** ability to run arbitrary language servers via
WebAssembly extensions. The functionality isn't exposed yet - we're just
landing this in this early state because there have been a lot of
changes to the `LspAdapter` trait, and other language server logic.

## Next steps

* Currently, wasm extensions can only define how to *install* and run a
language server, they can't yet implement the other LSP adapter methods,
such as formatting completion labels and workspace symbols.
* We don't have an automatic way to install or develop these types of
extensions
* We don't have a way to package these types of extensions in our
extensions repo, to make them available via our extensions API.
* The Rust extension API crate, `zed-extension-api` has not yet been
published to crates.io, because we still consider the API a work in
progress.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/extension_api/src/extension_api.rs

@@ -0,0 +1,62 @@
+pub struct Guest;
+pub use wit::*;
+
+pub type Result<T, E = String> = core::result::Result<T, E>;
+
+pub trait Extension: Send + Sync {
+    fn new() -> Self
+    where
+        Self: Sized;
+
+    fn language_server_command(
+        &mut self,
+        config: wit::LanguageServerConfig,
+        worktree: &wit::Worktree,
+    ) -> Result<Command>;
+}
+
+#[macro_export]
+macro_rules! register_extension {
+    ($extension_type:ty) => {
+        #[export_name = "init-extension"]
+        pub extern "C" fn __init_extension() {
+            zed_extension_api::register_extension(|| {
+                Box::new(<$extension_type as zed_extension_api::Extension>::new())
+            });
+        }
+    };
+}
+
+#[doc(hidden)]
+pub fn register_extension(build_extension: fn() -> Box<dyn Extension>) {
+    unsafe { EXTENSION = Some((build_extension)()) }
+}
+
+fn extension() -> &'static mut dyn Extension {
+    unsafe { EXTENSION.as_deref_mut().unwrap() }
+}
+
+static mut EXTENSION: Option<Box<dyn Extension>> = None;
+
+#[cfg(target_arch = "wasm32")]
+#[link_section = "zed:api-version"]
+#[doc(hidden)]
+pub static ZED_API_VERSION: [u8; 6] = *include_bytes!(concat!(env!("OUT_DIR"), "/version_bytes"));
+
+mod wit {
+    wit_bindgen::generate!({
+        exports: { world: super::Component },
+        skip: ["init-extension"]
+    });
+}
+
+struct Component;
+
+impl wit::Guest for Component {
+    fn language_server_command(
+        config: wit::LanguageServerConfig,
+        worktree: &wit::Worktree,
+    ) -> Result<wit::Command> {
+        extension().language_server_command(config, worktree)
+    }
+}