feat: add cssStub to jest

test/cssStub.ts

@@ -0,0 +1,5 @@
+// Note: this is needed for the register.test.ts
+// This is because inside src/browser/register.ts
+// we import CSS files, which Jest can't handle unless we tell it how to
+// See: https://stackoverflow.com/a/39434579/3015595
+module.exports = {}