Add changelogger to help generate changelogs on release

[joshuatf]

Currently changelogs are manually generated by looking at recently merged PRs at the time of the release.

This is error prone and may miss important changes in AS. We should consider adopting the Jetpack changelogger, also used in Woo core.

Acceptance Criteria

• Add Jetpack changelogger as a dependency
• Set up changelog script commands to generate a new entry
• Set up changelog script commands to generate the changelog from entries