Add dedicated package build and publish action

[dstansby]

This should fix https://github.com/matplotlib/napari-matplotlib/issues/317. I've simply followed the guide at https://packaging.python.org/en/latest/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows/ here, and removed the old workflow. In tandem, I've set up a new publishing environment on PyPI to go with the new workflow.

I think the only way we can test this is by merging this PR and doing a beta release...?