Github Action: cross-platfrom D compiler installation

[Mihails]

https://github.com/marketplace/actions/d-compiler-installation
https://github.com/mihails-strasuns/setup-dlang

Main difference from 
https://forum.dlang.org/thread/lrbcmnblvfplkpfaokkh@forum.dlang.org is that this is written using Github Action Typescript API instead of a docker container and thus allows testing with all platforms supported by Github Actions.

Example yml:

```
name: Test My D Project
on: [push, pull_request]

jobs:
     test:
         name: Test
         strategy:
             matrix:
                 os: [ubuntu-latest, windows-latest, macOS-latest]
                 dc: [dmd-2.088.0, ldc-1.17.0]
         runs-on: ${{ matrix.os }}
         steps:
             - uses: actions/checkout at master

             - name: Install D compiler
               uses: mihails-strasuns/setup-dlang at v0
               with:
                   compiler: ${{ matrix.dc }}

             - name: Run tests
               run: dub -q test
```

Example test run: 
https://github.com/mihails-strasuns/d-simpleconfig/commit/5107baedcb5db11cf0076e5c74e3f68594fb495b/checks

Currently only supports DMD and LDC with exact version 
specifiers. Supporting GDC and convenience bits like `latest` or 
`nightly` is planned but not currently implemented.