Switch from beachball to changesets

[grigasp]

beachball allows providing only a single paragraph per change without any markdown in it, so there's not much flexibility - we can't add examples, links, lists, etc.

Switching to changesets would provide the same versioning capabilities, but much better experience with changelogs.

For example, compare tree-widget-react CHANGELOG, generated with beachball, vs presentation-hierarchies CHANGELOG, generated with changesets.