extension_cli: Include the list of what an extension provides in the generated manifest (#24295)

This PR updates the Zed extension CLI with support for populating the
`provides` field in the generated extension manifest.

This field will contain the set of features that the extension provides.

For example:

```
"provides": ["themes", "icon-themes"]
```

Release Notes:

- N/A

crates/rpc/src/extension.rs

@@ -1,3 +1,5 @@
+use std::collections::BTreeSet;
+
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
@@ -11,6 +13,22 @@ pub struct ExtensionApiManifest {
     pub repository: String,
     pub schema_version: Option<i32>,
     pub wasm_api_version: Option<String>,
+    #[serde(default)]
+    pub provides: BTreeSet<ExtensionProvides>,
+}
+
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, Serialize, Deserialize)]
+#[serde(rename_all = "kebab-case")]
+pub enum ExtensionProvides {
+    Themes,
+    IconThemes,
+    Languages,
+    Grammars,
+    LanguageServers,
+    ContextServers,
+    SlashCommands,
+    IndexedDocsProviders,
+    Snippets,
 }
 
 #[derive(Clone, Serialize, Deserialize, Debug, PartialEq)]