Add support for unix, webassembly and windows ports

[mattytrentini]

The unix and webassembly ports are different since they don't have boards. They do, however, have variants - but they're listed differently, as a variants folder.

One way to handle this would be to use the port names as boards:

>mpbuild build unix
>mpbuild build webassembly pyscript

[Josverl]

Would love this, can you add a Windows build as well as that seems similar to the Unix and Webassembly builds

[mattytrentini]

I'd like to support Windows builds too, but that'll be a little more challenging since it won't work the same way with container builds. Still, may be possible...

[Josverl]

If it runs in GHA, it can run in a container,
So it can be a standard dev container with a call to tools/ci.sh

https://github.com/micropython/micropython/blob/master/tools%2Fci.sh#L722-L735

[mattytrentini]

Oh yes, we can definitely cross-compile to Windows (using mingw) but can't easily support VS/msbuild on Windows itself.

Sure, will add support for that.

[mattytrentini]

Mostly resolved with #30. Still need to add support for the containers for windows/webassembly builds.