build: ensure theming tests execute as ESM

The JS file is ESM, but Node doesn't recognize this because there is no
explicit extension or `package.json` with `type: module`.

Author: devversion

src/material/core/theming/tests/BUILD.bazel

@@ -72,6 +72,7 @@ jasmine_test(
     name = "unit_tests",
     data = [
         ":unit_test_lib",
+        "//src:package_json",
         "//src/material:sass_lib",
         "//src/material-experimental:sass_lib",
     ],