Add Windows as a CI build target

[brzpegasus]

We need to add support for Windows and not limit Beacon development to just MacOS and Linux machines.

At the moment, Beacon has a dependency on Vix, which is incompatible with Windows.

[leandrocp]

Some extra info about this problem.

Vix is an indirect dependency added by Image

❯ mix deps.tree image
image ~> 0.40 (Hex package)
└── vix ~> 0.23 (Hex package)

We use the Image library to perform some operations, including thumbnail generation and resize. Not many operations but that library has a low memory footprint and some advanced features like the crop: :attention option for thumbnails, so it's of our interest to keep using Image.