history rewrite to avoid release branches

[FND]

We kinda got ourselves into a tangle; see #164 (comment). Subsequent analysis resulted in consensus that the easiest way to restore a semblance of sanity would be to rewrite history:

• archive current main branch, just in case: archive-main-v3 (for lack of a better name?)
• archive current v3.0.0 tag, just in case: archive-v3.0.0 (for lack of a better name?)
• fix 🏷️ v2.1.0 to avoid accidentally diverging history
• rewrite main to include v2.1.0, restoring linear history
• deprecate v3.0.0, declaring it a mistake in hindsight - which shouldn't be an issue because the ecosystem (i.e. plugins) had not been upgraded yet anyway
• since we're rewriting history anyway, might as well release v2.2.0 while we're at it
• put 🏷️ v3.0.0 on top of all that - which means the repo's version is slightly different from what's out there on npm (though the differences merely constitute enhancements and should practically be imperceptible)

breaking changes will be re-released as v4.0.0