Add support for using a language server with multiple languages (#10293)

This PR updates the `extension.toml` to allow specifying multiple
languages for a language server to work with.

The `languages` field takes precedence over `language`. In the future
the `language` field will be removed.

As part of this, the Emmet extension has been extended with support for
PHP and ERB.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

crates/extension/src/extension_manifest.rs

@@ -98,11 +98,34 @@ pub struct GrammarManifestEntry {
 
 #[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
 pub struct LanguageServerManifestEntry {
-    pub language: Arc<str>,
+    /// Deprecated in favor of `languages`.
+    #[serde(default)]
+    language: Option<Arc<str>>,
+    /// The list of languages this language server should work with.
+    #[serde(default)]
+    languages: Vec<Arc<str>>,
     #[serde(default)]
     pub language_ids: HashMap<String, String>,
 }
 
+impl LanguageServerManifestEntry {
+    /// Returns the list of languages for the language server.
+    ///
+    /// Prefer this over accessing the `language` or `languages` fields directly,
+    /// as we currently support both.
+    ///
+    /// We can replace this with just field access for the `languages` field once
+    /// we have removed `language`.
+    pub fn languages(&self) -> impl IntoIterator<Item = Arc<str>> + '_ {
+        let language = if self.languages.is_empty() {
+            self.language.clone()
+        } else {
+            None
+        };
+        self.languages.iter().cloned().chain(language)
+    }
+}
+
 impl ExtensionManifest {
     pub async fn load(fs: Arc<dyn Fs>, extension_dir: &Path) -> Result<Self> {
         let extension_name = extension_dir