Add GHA workflow, allow to select shell variant, add checks, etc.

[eine]

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• The first commit in this PR is picked from branch releases/v1.
• @actions/core is updated to ^1.1.3.
• eslint is removed from package.json, because it is not usable without a configuration file.
• checks are added to ensure that the platform is windows (🤣) and that envvar RUNNER_TMP is not empty.
• A GitHub Actions (GHA) workflow is added to test the action after each push or PR.
• msysdo.cmd is first renamed to msysdo.in (for uses: ./ to work in the GHA workflow), but is then removed (by hardconding it in index.js).
• An option named msystem is added to the action, to allow users to select the variant of MSYS2 to use. It defaults to MINGW64.
• A test script (test.sh) is added and it is tested four times in the workflow: one with each allowed value of MSYSTEM and one without setting it explicitly.

[eine]

Superseded by #6.