PHP: expose file_filter via ls_specific_settings (#1710) (#1711)

- PHP/Intelephense: expose `file_filter` via `ls_specific_settings["php"]`, so that additional
  extensions containing PHP sources (e.g. Drupal's `.module` / `.install` / `.inc` / `.theme`)
  become visible to the symbol tools and are indexed by the language server
- PHP: treat `.phtml` files as PHP sources by default (all PHP language servers)

test/resources/repos/php/test_repo/drupal_module.module

@@ -0,0 +1,29 @@
+<?php
+
+/**
+ * @file
+ * Drupal-style module file: ordinary PHP kept in a non-.php extension.
+ *
+ * Only visible to the symbol tools when ls_specific_settings["php"]["file_filter"] includes
+ * ".module" (see #1710); inert for all tests running with default settings.
+ */
+
+/**
+ * Implements hook_help().
+ */
+function drupal_module_help($route_name) {
+    return 'Help for ' . $route_name;
+}
+
+/**
+ * Implements hook_cron().
+ */
+function drupal_module_cron() {
+    helperFunction();
+}
+
+class DrupalModuleController {
+    public function build(): string {
+        return drupal_module_help('help.page.drupal_module');
+    }
+}