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/wit/since_v0.0.6/settings.rs

@@ -0,0 +1,20 @@
+use serde::{Deserialize, Serialize};
+use std::num::NonZeroU32;
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct LanguageSettings {
+    pub tab_size: NonZeroU32,
+}
+
+#[derive(Default, Debug, Serialize, Deserialize)]
+pub struct LspSettings {
+    pub binary: Option<BinarySettings>,
+    pub initialization_options: Option<serde_json::Value>,
+    pub settings: Option<serde_json::Value>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct BinarySettings {
+    pub path: Option<String>,
+    pub arguments: Option<Vec<String>>,
+}