Build library and examples in CI

[Nerixyz]

GitHub CI is a simple way of ensuring most/all configurations of the library work.

This PR adds a CI job that tests the following configurations: examples disabled, examples enabled & Qt disabled, examples enabled & Qt enabled. The runs without Qt take about 1 min, the run with Qt takes about 4-5 min without a cache, but should be similar to the other ones with a cache.

For the used actions, you can optionally use dependabot to update them regularly:

# .github/dependabot.yml
version: 2
updates:
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "weekly"

[mohabouje]

Thanks for the pipeline, better safe than sorry!