[CLI] Add `release prepare` command

[jhnstn]

Adds the release prepare command.

The command prepares a Gutenberg release PR.

Testing

• SetGBM_WORDPRESS_ORG to a forked version of Gutenberg
  • This can be done inline with the command i.e. GBM_WORDPRESS_ORG=your_org go run main.go release prepare v1.0.0 or by setting the env in your shell config i.e. export GBM_WORDPRESS_ORG="your_org". The later is preferred if working on a lot of changes)
• run go run main.org release prepare {version} and answer the prompts to create a release pr on the forked repo

[jhnstn]

@derekblank I added a few more changes. I'm going to merge so we can move on to other work, Thanks for the review!