Fix issues with extension API that come up when moving Svelte into an extension (#9611)

We're doing it. Svelte support is moving into an extension. This PR
fixes some issues that came up along the way.

Notes

* extensions need to be able to retrieve the path the `node` binary
installed by Zed
* previously we were silently swallowing any errors that occurred while
loading a grammar
* npm commands ran by extensions weren't run in the right directory
* Tree-sitter's WASM stdlib didn't support a C function (`strncmp`)
needed by the Svelte parser's external scanner
* the way that LSP installation status was reported was unnecessarily
complex

Release Notes:

- Removed built-in support for the Svelte and Gleam languages, because
full support for those languages is now available via extensions. These
extensions will be suggested for download when you open a `.svelte` or
`.gleam` file.

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/extension/src/extension_builder.rs

@@ -85,10 +85,7 @@ impl ExtensionBuilder {
 
         let cargo_toml_path = extension_dir.join("Cargo.toml");
         if extension_manifest.lib.kind == Some(ExtensionLibraryKind::Rust)
-            || fs::metadata(&cargo_toml_path)
-                .ok()
-                .map(|metadata| metadata.is_file())
-                .unwrap_or(false)
+            || fs::metadata(&cargo_toml_path).map_or(false, |stat| stat.is_file())
         {
             log::info!("compiling Rust extension {}", extension_dir.display());
             self.compile_rust_extension(extension_dir, options)