Decompose changesets/cli into meaningful packages

[Noviny]

The goal: make it easy for people to build their own utilities with minimum fuss.

Current new packages:

@changesets/[command]

• programmatically run any of the changeset commands as a function if you have all the correct information.

[Noviny]

Not appearing in this thread are utils that will need to be shared between packages, as we will likely split these when we come to them.

[Noviny]

This change means commands need to be split between a single function that takes in info and does the action, and the bit that asks questions. Some of this logic is currently interleaved.