Switch the JS script used for running the docs examples tests from
CommonJS to ESM format. This is necessary for upgrading the `globby`
dependency to [version 12.0.0][1] in a subsequent commit.
[1]: https://github.com/sindresorhus/globby/releases/v12.0.0
PR Close#42921
Previously, the examples in the `comparing-observables` guide were hard-coded.
This made it impossible to test them and verify they are correct.
This commit fixes this by converting them into a proper mini-app. In a
subsequent commit, tests will be added to verify that the source code
works as expected (and guard against regressions).
Fixes#31024
PR Close#34327