added github workflow file

[joemarshall]

This is a simple github action which builds the wheel in pyodide build system and uploads it as a build artifact. Saves having to manually build it if you modify a fork.

[koenvo]

I'm not entirely sure in what case this helps. Could you elaborate on this?

[joemarshall]

It's useful because a) It's a sanity check that you're not doing anything that breaks the pyodide build, and b) if you want to fork and make a minor change you can just change it in your fork on github and download the build artifact without having to install anything at all (or know how to) build a pyodide package

It's most relevant on a package with C extensions, but I put this action into all my pyodide things just so I can see they build okay (and typically I chuck another action which does the release and adds the artifact to it, and one could also potentially auto push to pypi when you add a new version tag, so pypi always tracks github releases nicely)