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/extension.toml

@@ -0,0 +1,16 @@
+id = "php"
+name = "PHP"
+description = "PHP support."
+version = "0.0.1"
+schema_version = 1
+authors = ["Piotr Osiewicz <piotr@zed.dev>"]
+repository = "https://github.com/zed-industries/zed"
+
+[language_servers.intelephense]
+name = "Intelephense"
+language = "PHP"
+
+[grammars.php]
+repository = "https://github.com/tree-sitter/tree-sitter-php"
+commit = "8ab93274065cbaf529ea15c24360cfa3348ec9e4"
+path = "php"