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/CompletionSuggestionServiceProtocol.swift

@@ -0,0 +1,88 @@
+//
+// 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
+import Foundation
+import WysiwygComposer
+
+struct SuggestionTrigger: Equatable {
+    enum SuggestionType: Equatable {
+        case user
+    }
+    
+    let type: SuggestionType
+    let text: String
+    let range: NSRange
+}
+
+enum SuggestionItem: Identifiable, Equatable {
+    case user(item: MentionSuggestionItem)
+    case allUsers(item: MentionSuggestionItem)
+    
+    var id: String {
+        switch self {
+        case .user(let user):
+            return user.id
+        case .allUsers:
+            return PillConstants.atRoom
+        }
+    }
+    
+    var range: NSRange {
+        switch self {
+        case .user(let item), .allUsers(let item):
+            return item.range
+        }
+    }
+}
+
+struct MentionSuggestionItem: Identifiable, Equatable {
+    let id: String
+    let displayName: String?
+    let avatarURL: URL?
+    let range: NSRange
+}
+
+// sourcery: AutoMockable
+protocol CompletionSuggestionServiceProtocol {
+    var suggestionsPublisher: AnyPublisher<[SuggestionItem], Never> { get }
+    
+    func processTextMessage(_ textMessage: String?)
+    
+    func setSuggestionTrigger(_ suggestionTrigger: SuggestionTrigger?)
+}
+
+extension WysiwygComposer.SuggestionPattern {
+    var toElementPattern: SuggestionTrigger? {
+        switch key {
+        case .at:
+            return SuggestionTrigger(type: .user, text: text, range: .init(location: Int(start), length: Int(end)))
+        default:
+            return nil
+        }
+    }
+}
+
+extension CompletionSuggestionServiceMock {
+    struct CompletionSuggestionServiceMockConfiguration {
+        var suggestions: [SuggestionItem] = []
+    }
+    
+    convenience init(configuration: CompletionSuggestionServiceMockConfiguration) {
+        self.init()
+        underlyingSuggestionsPublisher = Just(configuration.suggestions).eraseToAnyPublisher()
+    }
+}