Added GitHub Actions

[samsinsane]

What does this PR do?

This adds GitHub Actions to do CI as a potential replacement for both Travis-CI and AppVeyor.

How does this PR change Premake's behavior?

N/A.

Anything else we should know?

Windows builds use windows-msbuild instead of windows, the upgrade is unnecessary.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits

[starkos]

Ramping up on Workflows now, approving in the meantime.

[samsinsane]

Hmm, it doesn't appear to have done a master build, I must have got something wrong. Pushing branches worked in my fork, and I assume PRs will work since it does specify them the same way as pushing, I guess neither are merges? I'll look into this when I get a chance 👍

Edit: I guess I spoke too soon, the PR I just merged triggered a CI build.