Make Get helper interop with DT and preview types

In the `intrinsics` module for the `@glint/environment-ember-loose`,
add a declaration for the `ComputedPropertyMarker`. If using the DT
types, this acts as a declaration merge for a module which is then
integrated via the rest of the DT types. If using the stable/preview
types, this ends up being a no-op.

packages/environment-ember-loose/-private/intrinsics/get.d.ts

@@ -1,6 +1,15 @@
 import { DirectInvokable } from '@glint/template/-private/integration';
 import ObjectProxy from '@ember/object/proxy';
-import { UnwrapComputedPropertyGetter } from '@ember/object/-private/types';
+import '@ember/object/-private/types';
+
+declare const GetSetMarker: unique symbol;
+declare module '@ember/object/-private/types' {
+  interface ComputedPropertyMarker<Get, Set = Get> {
+    [GetSetMarker]: [Get, Set];
+  }
+}
+
+type UnwrapComputedPropertyGetter<T> = T extends { [GetSetMarker]: [infer U, any] } ? U : T;
 
 export type GetHelper = DirectInvokable<{
   <T, K extends keyof T>(obj: T, key: K): UnwrapComputedPropertyGetter<T[K]>;