Add language_server_workspace_configuration to extension API (#10212)

This PR adds the ability for extensions to implement
`language_server_workspace_configuration` to provide workspace
configuration to the language server.

We've used the Dart extension as a motivating example for this, pulling
it out into an extension in the process.

Release Notes:

- Removed built-in support for Dart, in favor of making it available as
an extension. The Dart extension will be suggested for download when you
open a `.dart` file.

---------

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

crates/extension_api/src/settings.rs

@@ -0,0 +1,30 @@
+#[path = "../wit/since_v0.0.6/settings.rs"]
+pub mod types;
+
+use crate::{wit, Result, SettingsLocation, Worktree};
+use serde_json;
+pub use types::*;
+
+impl LanguageSettings {
+    pub fn for_worktree(language: Option<&str>, worktree: &Worktree) -> Result<Self> {
+        let location = SettingsLocation {
+            worktree_id: worktree.id(),
+            path: worktree.root_path(),
+        };
+        let settings_json = wit::get_settings(Some(&location), "language", language)?;
+        let settings: Self = serde_json::from_str(&settings_json).map_err(|err| err.to_string())?;
+        Ok(settings)
+    }
+}
+
+impl LspSettings {
+    pub fn for_worktree(language_server_name: &str, worktree: &Worktree) -> Result<Self> {
+        let location = SettingsLocation {
+            worktree_id: worktree.id(),
+            path: worktree.root_path(),
+        };
+        let settings_json = wit::get_settings(Some(&location), "lsp", Some(language_server_name))?;
+        let settings: Self = serde_json::from_str(&settings_json).map_err(|err| err.to_string())?;
+        Ok(settings)
+    }
+}