feat(supported-version): validate custom_versions

Previously, @danslo reported that he tried to use `custom_versions` without
setting the kind. This isn't the correct behavior.
I've added a validator to alert him of this.

supported-version/src/kind/validate-kinds.spec.ts

@@ -0,0 +1,15 @@
+import { validateKind } from "./validate-kinds";
+
+describe('validateKind', () => {
+    it('returns `true` if its a valid kind', () => {
+        expect(validateKind("latest")).toBe(true);
+    });
+
+    it('throws a helpful exception if its an invalid kind', () => {
+        expect(() => validateKind(<any>"taco")).toThrowError();
+    })
+
+    it('throws a helpful exception if custom versions are provided with the wrong kind', () => {
+        expect(() => validateKind(<any>"latest", [])).toThrowError();
+    })
+})