* .github/workflows: new CI pipeline
As Azure Pipelines went offline, this will be our (temporary?)
replacement. This pipeline covers everything that Azure Pipelines does.
* workflows/ci: disable pipeline
* workflows/ci: properly disable the pipeline
* gitattributes: .txt used in tests should be LF-terminated
This avoids test failures when the compiler was cloned on an OS where
git attempts to convert LF->CRLF.
We target only .txt in tests/ as the minimum required to keep tests
running correctly.
* workflows/ci: re-enable for testing
