Create trainhop skill group under `plugins/newtab`

[nathan-barrett]

Create separate sub-skills for each individual task:

1. Bump minor version
2. Update metrics/pings — mach newtab channel-metrics-diff --channel release/beta, commit result
3. Find & file @backward-compat shims — grep codebase for @backward-compat { version NNN }, file Bugzilla bugs (and optionally fix them)
4. Update locales — ./mach newtab update-locales, show report, commit