Implement MSC3952: intentional mentions (#9983)

Implements the intentional mentions feature of MSC3952 (behind
a labs flag).

If enabled, this will send an org.matrix.msc3952.mentions property
on events that will contain the user IDs and/or whether the room is
being mentioned. These mentions also gets propagated via some
custom behaviour for replies and edits.

src/settings/Settings.tsx

@@ -540,6 +540,17 @@ export const SETTINGS: { [setting: string]: ISetting } = {
         labsGroup: LabGroup.Rooms,
         default: false,
     },
+    // MSC3952 intentional mentions support.
+    "feature_intentional_mentions": {
+        isFeature: true,
+        supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS_WITH_CONFIG,
+        displayName: _td("Enable intentional mentions"),
+        labsGroup: LabGroup.Rooms,
+        default: false,
+        controller: new ServerSupportUnstableFeatureController("feature_intentional_mentions", defaultWatchManager, [
+            ["org.matrix.msc3952_intentional_mentions"],
+        ]),
+    },
     "useCompactLayout": {
         supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS,
         displayName: _td("Use a more compact 'Modern' layout"),