Note: currently a warning is emitted when building, because dscanner now
depends on libdparse 0.22.0, but dsymbol doesn't support it officially
yet. We just force it with dub.selections.json to build with 0.22.0 for
executable builds.
* Add missing files + import paths for libddoc/common to build.bat
* Add Github Actions setup to replace Travis
The configured workflow tests D-Scanner with different host compilers
(DMD / LDC / GDC) on different platforms ( Windows / OSX / Linux).
The tests include
- `build.bat test` (windows) or `make test`
- `dub test` for version ranges of `libdparse` and `dsymbol`
(currently disabled because of unresolvable dependency constraints)
- `make lint`
- phobos linting
