php: Add Phpactor support (#14604)

This PR extends the PHP extension with
[Phpactor](https://github.com/phpactor/phpactor) support.

Phpactor seems to provide a better feature set out-of-the-box for free,
so it has been made the default PHP language server.

Thank you to @xtrasmal for informing us of Phpactor's existence!

Release Notes:

- N/A

extensions/php/src/language_servers.rs

@@ -0,0 +1,5 @@
+mod intelephense;
+mod phpactor;
+
+pub use intelephense::*;
+pub use phpactor::*;