Set up automated tests and static analysis on CI #7

Job	Run time
Static analysis \| sass_language_services	17s
Dart tests \| Dart stable \| ubuntu-latest \| sass_language_services	20s
VS Code tests \| ubuntu-latest	43s
Static analysis \| sass_language_server	19s
Dart tests \| Dart stable \| ubuntu-latest \| sass_language_server	20s
VS Code tests \| windows-latest	1m 41s
Dart tests \| Dart stable \| windows-latest \| sass_language_services	34s
VS Code tests \| macos-latest	49s
Dart tests \| Dart stable \| windows-latest \| sass_language_server	36s
Dart tests \| Dart stable \| macos-latest \| sass_language_services	23s
Dart tests \| Dart stable \| macos-latest \| sass_language_server	21s
	6m 23s

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