Early implementation of MSC3089 (file trees)

MSC: https://github.com/matrix-org/matrix-doc/pull/3089
Includes part of MSC3088 (room subtyping): https://github.com/matrix-org/matrix-doc/pull/3088

The NamespacedValue stuff is borrowed from the Extensible Events implementation PR in the react-sdk as a useful thing to put here. When/if the MSCs become stable, we'd convert the values to enums and drop the constants (or keep them for migration purposes, but switch to stable). 

This flags the whole thing as unstable because it's highly subject to change.

src/NamespacedValue.ts

@@ -0,0 +1,95 @@
+/*
+Copyright 2021 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.
+*/
+
+/**
+ * Represents a simple Matrix namespaced value. This will assume that if a stable prefix
+ * is provided that the stable prefix should be used when representing the identifier.
+ */
+export class NamespacedValue<S extends string, U extends string> {
+    public constructor(public readonly stable: S, public readonly unstable?: U) {
+        if (!this.unstable && !this.stable) {
+            throw new Error("One of stable or unstable values must be supplied");
+        }
+    }
+
+    public get tsType(): U | S {
+        return null; // irrelevant return
+    }
+
+    public get name(): U | S {
+        if (this.stable) {
+            return this.stable;
+        }
+        return this.unstable;
+    }
+
+    public get altName(): U | S | null {
+        if (!this.stable) {
+            return null;
+        }
+        return this.unstable;
+    }
+
+    public matches(val: string): boolean {
+        return this.name === val || this.altName === val;
+    }
+
+    // this desperately wants https://github.com/microsoft/TypeScript/pull/26349 at the top level of the class
+    // so we can instantiate `NamespacedValue<string, _, _>` as a default type for that namespace.
+    public findIn<T>(obj: any): T {
+        let val: T;
+        if (this.name) {
+            val = obj?.[this.name];
+        }
+        if (!val && this.altName) {
+            val = obj?.[this.altName];
+        }
+        return val;
+    }
+
+    public includedIn(arr: any[]): boolean {
+        let included = false;
+        if (this.name) {
+            included = arr.includes(this.name);
+        }
+        if (!included && this.altName) {
+            included = arr.includes(this.altName);
+        }
+        return included;
+    }
+}
+
+/**
+ * Represents a namespaced value which prioritizes the unstable value over the stable
+ * value.
+ */
+export class UnstableValue<S extends string, U extends string> extends NamespacedValue<S, U> {
+    // Note: Constructor difference is that `unstable` is *required*.
+    public constructor(stable: S, unstable: U) {
+        super(stable, unstable);
+        if (!this.unstable) {
+            throw new Error("Unstable value must be supplied");
+        }
+    }
+
+    public get name(): U {
+        return this.unstable;
+    }
+
+    public get altName(): S {
+        return this.stable;
+    }
+}