[Proposal] Refactor ng-dev's pr directory to disentangle the utilities and common pieces

[josephperrott]

Currently we have a structure like this.

ng-dev/pr                   
├── checkout                │ Checks out a PR locally to modify
├── check-target-branches   │ Prints the branches a pr would merge into
├── common                  │ 
│   └── checkout-pr.ts      │ Actual logic for checking out a PR used by checkout and rebase
├── discover-new-conflicts  │ Determines what other pending PRs can merge cleanly after a PR merges
├── merge                   │ Merges PRs (also houses the config.ts)
└── rebase                  │ Attempts to rebase a PR and push the rebased commits back to upstream

We have had a few people ask for something akin to ng-dev pr check <pr-number>, which would return the pr title, number, if it needs approvals, what branches it would land in etc. I think we should transform the check-target-branches into this command.

I would propose that we then separate out a few pieces that can be reused both literally in other files/commands and conceptually.

ng-dev/pr                   
├── checkout                │ Stays the same
├── info                    │ Leverage the validation logic and PullRequest object to get all PR info
├── common                  │ 
│   ├── checkout-pr         │ Stays the same
│   ├── get-pr              │ Common util to get PR from github as a consistent PullRequest object
│   ├── targeting           │ Target labels and utils to get a target branch for a PR, etc
│   └── validation          │ Validate that a PR meets all of the expected criteria for the repo
├── discover-new-conflicts  │ Stays the same
├── merge                   │ Merges PR, using the new common utils
└── rebase                  │ Stays the same

• config.ts should move up one level into ng-dev/pr
• MergeConfig should be renamed to something like PullRequestConfig

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