Wire up module loading to application startup (#21703)

* Early module loader bundler

* Add a module installer script

* Add dev-friendly docs

* Add real module-api dependency

* Speed up `yarn add` for mulitple modules

* Fix version check for modules

* Appease the linter

module_system/BuildConfig.ts

@@ -0,0 +1,33 @@
+/*
+Copyright 2022 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 * as YAML from "yaml";
+import * as fs from "fs";
+
+export type BuildConfig = {
+    // Dev note: make everything here optional for user safety. Invalid
+    // configs are very possible.
+
+    // The module references to include in the build.
+    modules?: string[];
+};
+
+export function readBuildConfig(): BuildConfig {
+    if (fs.existsSync("./build_config.yaml")) {
+        return YAML.parse(fs.readFileSync("./build_config.yaml", "utf-8"));
+    }
+    return {}; // no config
+}