Make :StartReview command auto-detect the PR number

[ReeceStevens]

Currently, to start a review you have to manually provide the PR number. This shouldn't be necessary since we have the remote URL and the branch name.

Requirements:

1. First time :StartReview without arguments is used on a branch, query GitHub API for open PRs for this branch. If only one, auto-select and print the PR number used as a message. If more than one (e.g. different base branches), print all options and wait for user input to select which one.

2. For future usages of :StartReview without arguments, check the local review files first to find any ongoing review for this branch. Same rules for single or multiple apply.

3. If no matching local reviews are found, AND the GH API is inaccessible (e.g. offline review), prompt the user for a PR number to be manually entered.