Extract PHP support into an extension (#9966)

This PR extracts PHP support into an extension and removes the built-in
PHP support from Zed.

There's a small workaround necessary in order for us to provide the
`language_ids` on the `LspAdapter` that are needed for the language
server to run properly. Eventually we'll want to build this into the
extension API, but for now we're just hard-coding it on the host side.

Release Notes:

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

extensions/php/languages/php/embedding.scm

@@ -0,0 +1,36 @@
+(
+    (comment)* @context
+    .
+    [
+        (function_definition
+            "function" @name
+            name: (_) @name
+            body: (_
+                "{" @keep
+                "}" @keep) @collapse
+            )
+
+        (trait_declaration
+            "trait" @name
+            name: (_) @name)
+
+        (method_declaration
+            "function" @name
+            name: (_) @name
+            body: (_
+                "{" @keep
+                "}" @keep) @collapse
+            )
+
+        (interface_declaration
+            "interface" @name
+            name: (_) @name
+            )
+
+        (enum_declaration
+            "enum" @name
+            name: (_) @name
+            )
+
+        ] @item
+    )