Bump minimum Swift to 5.9, adopt StrictConcurrency=complete, add GitHub Actions CI #1

Job	Run time
Soundness / API breakage check	4m 13s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	43s
Soundness / Format check	41s
Soundness / License headers check	9s
Soundness / Python lint check	8s
Soundness / Shell check	40s
Soundness / Unacceptable language check	5s
Soundness / YAML lint check	4s
Cxx interop / Cxx interop / Windows (6.0)	2s
Cxx interop / Cxx interop / Linux (5.9)	2m 31s
Cxx interop / Cxx interop / Linux (5.10)	2m 29s
Cxx interop / Cxx interop / Linux (6.0)	3m 1s
Cxx interop / Cxx interop / Windows (nightly-6.0)	1s
Cxx interop / Cxx interop / Linux (nightly-6.0)	3m 27s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 29s
Cxx interop / Cxx interop / Windows (nightly-main)	10s
Unit tests / Unit tests / Linux (5.9)	2m 38s
Unit tests / Unit tests / Windows (6.0)	1s
Unit tests / Unit tests / Windows (nightly-6.0)	3s
Unit tests / Unit tests / Linux (5.10)	2m 24s
Unit tests / Unit tests / Windows (nightly-main)	2s
Unit tests / Unit tests / Linux (6.0)	6h 0m 1s
Unit tests / Unit tests / Linux (nightly-6.0)	6h 0m 1s
Unit tests / Unit tests / Linux (nightly-main)	6h 0m 1s
	18h 27m 7s

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