proper per-branch CD

[ilonachan]

This is to improve upon the existing on-push CD pipeline for the repo. Currently it only pushes the master branch to plugins/v${VERSION}; there was apparently an attempt to make this work for "all branches" in forks, but it doesn't work for me at all (partly because of weird secret handling).

So this version vastly simplifies the process:

• No need to do anything with secrets manually, in workflows ${{ github.token }} can just be used to get access to the repo
• The pipeline runs upon push to any branch, but it only checks out and operates on that specific branch
• For every branch, the result will be pushed to a separate branch dist/$branchname. But if the current branch is master, the result will also be published at plugins/v$version like before.
• The pipeline previously wouldn't have ran if just the version changed, but now it will. Additionally, if bundlers ever become relevant, then version changes in dependencies could also mean actual changes to the plugin bundles and would need to retrigger the pipeline.
• And in case something goes wrong, the pipeline can in theory be retriggered manually now.