In working on #2313 it struck me that the pr create command has become unmaintainable:

• over 700 lines of code spread across 3-4 massive functions
• full of mutually exclusive state: interactive vs. autofill, webmode vs. not
• spaghetti'd: there is a tremendous amount of state required for it to work and it's not possible
  for a human to keep it in their head

This PR has these goals:

• PR creation code is comprehensible by the human brain
• No massive spaghetti functions anymore
• Composable functions usable by other commands and by issue creation code

The only "functional" change I ended up with is not computing defaults in cases where we are definitely not going to use them (ie when both title and body have already been provided via CLI args).