Add mingw to supported platforms

[rolandlo]

Having installed lgi through luarocks in a MinGW 64-bit console and used it for various demos successfully I suggest to add mingw to the list of supported platforms in the rockspec, so Windows users can install the lgi rock without any modifications to the repo through luarocks.

When testing the various demos in gtk-demo all appeared to work except of the Images and Rotated Button demos. I have also tested some Xournal++ demos that use lgi and they worked too.

Some more infos on how to install can be found in this discussion. Note that for running gtk-demo you also have to install gtksourceview3.

[rolandlo]

And here is a screenshot.

[psychon]

The tests are failing due to Ubuntu now using more bits for ASLR and that causes conflicts with some hard coded address in ASAN. E.g. https://bugs.launchpad.net/ubuntu/+source/llvm-toolchain-14/+bug/2048768 has some more information (and links to even more infos in a comment from yesterday - that wasn't there the last time I looked at this).

ANyway, this looks obvious enough that I am going ahead and just merge this.

[psychon]

Ah, sorry, due to all the test debugging I forgot what I initially wanted to say:

Thanks for the PR, thanks for doing all the work of testing this and thanks for the nice write up. Oh and thanks for linking to it. That all is really convincing and I'm happy to see such a high quality report instead of just a It works.