ci: setup `semantic-release`

[G-Rath]

This should work out of the box, but got to do some testing with a few commits to ensure all the checks are happening.

@schwamster this'll require a GH access token to be added to the env with write permissions, to allow semantic-release to commit back changelog and version bumping, as well as do the github release.

I also recommend making squashing the default merge strategy for the repo, as it works a lot nicer with this flow.