setup-dmd GitHub action

[WebFreak001]

I made a GitHub Actions action and published it on the GitHub 
marketplace which sets up DMD on Windows, Linux and OSX in the 
virtual environment on a GitHub Actions runner.

It supports any stable release (theoretically also below 2.064) 
and any pre-release denoted with the -beta.1 suffix for example. 
It can also use the nightly builds from the downloads page and 
it's very easy to use.

I first tried to use the dlang runner which has an open PR on the 
actions repository right now but it turned out it limits it to 
running as docker and only ubuntu, so I instead made this 
typescript action which runs on all operating systems and uses 
the APIs to download & cache the tools so the builds are really 
quick.

Marketplace: https://github.com/marketplace/actions/setup-dmd
GitHub: https://github.com/WebFreak001/setup-dmd

(well while writing this forum post I saw that there is another 
project which does the same but now it's already too late, I did 
search for something when I started working on this when it 
wasn't there yet :p)