Plain composer suggestions and pills (#2751)

* Move ComposerToolbar files to withing the RoomScreen folder (as it's not a screen on its own)

* Switch the plain composer to NSAttributedStrings

* Enable suggestions on the plain composer

* Introduce a new `MatrixUserDisplayName` attributed string attributed and use it for mention building

* Implement mention pill rendering and sending on the plain message composer

* Fix plain composer snapshot tests

* Fix broken formatting options layout

* Add clarifying comment.

ElementX/Sources/Screens/RoomScreen/ComposerToolbar/ComposerToolbarViewModelProtocol.swift

@@ -0,0 +1,24 @@
+//
+// Copyright 2023 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import Combine
+
+// periphery: ignore - markdown protocol
+protocol ComposerToolbarViewModelProtocol {
+    var actions: AnyPublisher<ComposerToolbarViewModelAction, Never> { get }
+    var context: ComposerToolbarViewModelType.Context { get }
+    func process(roomAction: RoomScreenComposerAction)
+}