Support package-level tags

[mcmire]

Currently when looking for tags, we assume that all tags start with v:

auto-changelog/src/update-changelog.ts

Line 19 in 1c931ec

assert.equal(mostRecentTag?.[0], 'v', 'Most recent tag should start with v');

We want this to be configurable. Perhaps we accept a tagPrefix option that's an array of prefixes to try.