This commit adds a GitHub Actions workflow that, on every push/pull
request, will check that every single example successfully compiles on
every board it supports. That is, it it will check compilation on:
- ESP8266 for BASIC_v4, TestCO2, TestPM and TestSht
- ESP32 for ONE_I-9PSL, Open_Air, TestCO2, TestPM and TestSht
This provides the first building block towards a Continuous Integration
(CI) pipeline and prevents build breakages from making it to master.
Ideally this should also run tests on the examples (i.e. verifying that
the example boots successfully and sends metrics), but for now this will
at least ensure the build is not obviously broken.
