Initial CI support

[koppor]

This is a setup to solve https://github.com/reitzig/ltx2any/issues/116.

1. Run tests against
* Ruby versions 2.3, 2.4, 2.5;

Simple execution done with these versions. Real LaTeXing test only Ruby 2.5, because even that is outdated now (see https://gitlab.alpinelinux.org/alpine/aports/-/issues/13555#note_218801)

* GNU/Linux and TeXlive;

Done via https://github.com/reitzig/texlive-docker

* macOS and MacTeX (?);

Note done; one could follow the steps at https://github.com/actions/virtual-environments/issues/1143#issuecomment-652264388.

* Windows and MiKTeX/proTeXt (?).

With Docker hard to do https://github.com/actions/virtual-environments/issues/1143#issuecomment-651670345; even though MiKTeX offers a Docker image.

2. Build and deploy docs (if applicable).

Not done.

3. Build and deploy gem (if commit tagged).

Not done. Could be done with a Google Cloud Platform action: https://andrewm.codes/blog/automating-ruby-gem-releases-with-github-actions/

When this update is in place, the output on GitHub is as follows:

I don't know whether this works with chew (https://github.com/reitzig/ltx2any/tree/gemification), but I think, this can be adapted when the development is continued.

Side comment: The motivation to look into this is that installing ruby on a Docker image is less a hazzle than ensuring that makeglossaries is called at the correct place in other build tools. cluttex was my choice after latexmk, because it is written in lua, which is available in LaTeX installations. However, it does trigger bibtex when a .bib file is updated. -- Maybe, a stand-alone binary can be generated in the future so that one does not need to play around with ruby versions.