feat(@angular-devkit/core): add DeepReadonly<> and Readwrite<> types

Readwrite removes readonly, while DeepReadonly recursively sets it. A ReadonlyArray
should be used if the type is an array, but that is too much of a change to our API
(JsonValue includes JsonArray but then we would need a ReadonlyJsonValue, and same
for object, which has deep roots in our API).

We should fix this in a further refactor at some point.

packages/angular_devkit/core/src/utils/index.ts

@@ -16,3 +16,20 @@ export * from './priority-queue';
 export * from './lang';
 
 export { tags, strings };
+
+export type DeepReadonly<T> =
+  T extends (infer R)[] ? DeepReadonlyArray<R> :
+    T extends Function ? T :
+      T extends object ? DeepReadonlyObject<T> :
+        T;
+
+// This should be ReadonlyArray but it has implications.
+export interface DeepReadonlyArray<T> extends Array<DeepReadonly<T>> {}
+
+export type DeepReadonlyObject<T> = {
+  readonly [P in keyof T]: DeepReadonly<T[P]>;
+};
+
+export type Readwrite<T> = {
+  -readonly [P in keyof T]: T[P];
+};