Add prerelease workflow

[daogrady]

We currently have a rather unreliable release cycle and sometimes wait for several features to be completed before we bundle them into one release. This can be cumbersome to users who wait for a particular feature to be released. This PR therefore introduces a workflow that automatically pushes a new -beta version to npmjs whenever changes land in main. Prereleases can be skipped by adding [skip prerelease] to the merge commit head, to avoid flooding users with new betas for every tiny change.

Caveat: if we decide to employ this workflow we would have to always bump the package version to the next expected number right after a release. We currently only bump the version number right before a release. But since the version number for the prereleases are derived from the current version found in package.json, that would create a beta version for the last released version, which doesn't make sense.