Introduce ambient type definitions

Ultimately we'd like to convert the package to TS, but this unblocks
wider ecosystem adoption of the stable/preview Ember types. This PR can
also serve as a model for other packages which want to publish types in
a similar way.

This will also work just fine with the `@types` packages from
DefinitelyTyped once [a key PR][pr] lands there, but that is not the
path we will recommend, so this only includes the `ember-source` peer
dependency.

[pr]: DefinitelyTyped/DefinitelyTyped#63589

addon/index.d.ts

@@ -0,0 +1,2 @@
+import { Resolver as ResolverContract } from "@ember/owner";
+export default interface Resolver extends Required<ResolverContract> {}

addon/package.json

@@ -9,6 +9,7 @@
   "bugs": {
     "url": "https://github.com/ember-cli/ember-resolver/issues"
   },
+  "types": "index.d.ts",
   "repository": {
     "type": "git",
     "url": "git+https://github.com/ember-cli/ember-resolver.git"
@@ -25,6 +26,14 @@
     "ember-cli-babel": "^7.26.11"
   },
   "devDependencies": {},
+  "peerDependencies": {
+    "ember-source": "^4.8.3"
+  },
+  "peerDependenciesMeta": {
+    "ember-source": {
+      "optional": true
+    }
+  },
   "engines": {
     "node": "14.* || 16.* || >= 18"
   },