ci: ensure example/ passes renovate-config-validator

As noted in #910, we have invalid configuration shared as an example,
which isn't ideal.

We should validate any JSON and `config.js` files in the example
directory.

We need to make sure that we override the ESM settings in this repo, as
otherwise we receive:

  module is not defined in ES module scope

When trying to validate the `renovate-config.js`.

This also follows the usage of a container from Renovate[0] for using
the container to run the validator.

[0]: 2ab0f77818/.github/workflows/test.yml (L44)

example/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "commonjs"
+}