Ignore non-ruby files in file watcher (#1912)

Currently there is only a listener for ruby files but that might changein the future.
Addons are also able to register additional file watcher events, config files as an example

lib/ruby_lsp/server.rb

@@ -610,6 +610,7 @@ def workspace_did_change_watched_files(message)
         uri = URI(change[:uri])
         file_path = uri.to_standardized_path
         next if file_path.nil? || File.directory?(file_path)
+        next unless file_path.end_with?(".rb")
 
         load_path_entry = $LOAD_PATH.find { |load_path| file_path.start_with?(load_path) }
         indexable = RubyIndexer::IndexablePath.new(load_path_entry, file_path)

test/server_test.rb

@@ -474,6 +474,27 @@ def test_backtrace_is_printed_to_stderr_on_exceptions
     assert_match(%r{ruby-lsp/lib/ruby_lsp/server\.rb:\d+:in `process_message'}, stderr)
   end
 
+  def test_changed_file_only_indexes_ruby
+    @server.global_state.index.expects(:index_single).once.with do |indexable|
+      indexable.full_path == "/foo.rb"
+    end
+    @server.process_message({
+      method: "workspace/didChangeWatchedFiles",
+      params: {
+        changes: [
+          {
+            uri: URI("file:///foo.rb"),
+            type: RubyLsp::Constant::FileChangeType::CREATED,
+          },
+          {
+            uri: URI("file:///.rubocop.yml"),
+            type: RubyLsp::Constant::FileChangeType::CREATED,
+          },
+        ],
+      },
+    })
+  end
+
   private
 
   def with_uninstalled_rubocop(&block)