Include `myst-parser` in list of provided extensions

[ckittl]

Thanks a lot @trustin for your work! I really enjoy using your sphinx-gradle-plugin! I typically use it to locally check, if my changes work out well when deployed to readthedocs.org.

However, they recommend using myst-parser instead of recommonmark (cf. documentation). Therefore, it would be really beneficial to include it in the list of available plugins as well.

[trustin]

Hey, I've just pushed a new commit 1abe4b5 to add myst-parser to pre-installed extensions. Thanks for your PR and sorry I didn't take a look into it on time. I need to figure out how to set up GitHub Actions to do the same job (uploading binaries, etc) with my old TravisCI configuration. But meanwhile, please clone the repository and build it yourself and let me know if it works! I tested it in my Linux machine and I successfully replaced recommonmark with myst_parser. 🙇