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

docs/src/languages/php.md

@@ -1,3 +1,21 @@
 # PHP
 
 PHP support is available through the [PHP extension](https://github.com/zed-industries/zed/tree/main/extensions/php).
+
+## Choosing a language server
+
+The PHP extension offers both `phpactor` and `intelephense` language server support.
+
+`phpactor` is enabled by default.
+
+To switch to `intelephense`, add the following to your `settings.json`:
+
+```json
+{
+  "languages": {
+    "PHP": {
+      "language_servers": ["intelephense", "!phpactor", "..."]
+    }
+  }
+}
+```