add feature set to keep track of enabled features

already include the calls feature

src/features.ts

@@ -0,0 +1,50 @@
+/*
+Copyright 2023 The Matrix.org Foundation C.I.C.
+
+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 type {SettingsStorage} from "./platform/web/dom/SettingsStorage";
+
+export enum FeatureFlag {
+    Calls = 1 << 0,
+}
+
+export class FeatureSet {
+    constructor(public readonly flags: number = 0) {}
+
+    withFeature(flag: FeatureFlag): FeatureSet {
+        return new FeatureSet(this.flags | flag);
+    }
+
+    withoutFeature(flag: FeatureFlag): FeatureSet {
+        return new FeatureSet(this.flags ^ flag);
+    }
+    
+    isFeatureEnabled(flag: FeatureFlag): boolean {
+        return (this.flags & flag) !== 0;
+    }
+
+    get calls(): boolean {
+        return this.isFeatureEnabled(FeatureFlag.Calls);
+    }
+
+    static async load(settingsStorage: SettingsStorage): Promise<FeatureSet> {
+        const flags = await settingsStorage.getInt("enabled_features") || 0;
+        return new FeatureSet(flags);
+    }
+
+    async store(settingsStorage: SettingsStorage): Promise<void> {
+        await settingsStorage.setInt("enabled_features", this.flags);
+    }
+}