feat: allow using "next" version on supported version (#58)

This also allows using the action in https://github.com/mage-os/generate-mirror-repo-js where the test module is an external package installed with composer. In this case, no local path repo configuration is needed.

Co-authored-by: Vinai Kopp <Vinai@users.noreply.github.com>

supported-version/src/nightly/get-next-version.spec.ts

@@ -0,0 +1,12 @@
+import { getNextVersion } from "./get-next-version"
+
+describe('getNextVersion', () => {
+
+    it('should get the next nightly version for MageOS', () => {
+        expect(getNextVersion('https://upstream-mirror.mage-os.org', new Date('2022-09-29T17:47:00')), ).toEqual('@alpha');
+    });
+
+    it('should handle the first of the month correctly', () => {
+        expect(getNextVersion('https://upstream-mirror.mage-os.org', new Date('2022-01-01T17:47:00')), ).toEqual('@alpha');
+    });
+})