Add support for action.yaml

[FottyM]

It would be nice to support also action.yaml instead of just yml, though it is the same file format, and it is just one character less. However, I am pretty sure there are some people out there who prefer yaml to yml, or just type it like that by muscle memory :)

[Roms1383]

I think @FottyM is right.

I created a small github action and couldn't see the checkbox "publish to github marketplace".
I wrote to github support and they rightfully suggested me to rename the file to action.yaml instead. If I do, I can now publish to the marketplace, but then I get a failure with JasonEtco/build-and-tag-action, as seen below.