Add CI based releases

[stevejpurves]

The last release of thebe likely contained a locally patched set of styles for ipywidgets by mistake, moving to a CI based release cycle will avoid this kind of error in future.

[stevejpurves]

Added an initial action to use the action provided by the changesets package as documented here https://github.com/changesets/action - to go further and publish from the action we need to be able to publish with the rc tag on the thebe package. Also consider setting up a separate dev branch if we are releasing from main.