✨ Add curated list of subcommands to core Short - #1952

Adds cmdutil.ApplyCuratedShort which accepts a command, a resource name (e.g: Pull Requests) and a list of subcommand names which are then used to generate a human friendly list of subcommands supported for the Short description of the command.

Note: this is a speculative example to be reviewed by @ampinsk -- in response to their comment on #1777. The PR is ready to go but marked as Draft so it isn't merged before @ampinsk has decided if this is the right direction :-)

CORE COMMANDS
  gist:       create, view and edit Gists (+1 more)
  issue:      create, view and close Issues (+3 more)
  pr:         create, review and merge Pull Requests (+9 more)
  release:    create, view and download Releases (+3 more)
  repo:       create, fork and garden Repositories (+3 more)

ApplyCuratedShort is explicitly added to a command after the subcommands have been registered, e.g:

//                              Resource             Curated list of commands
cmdutil.ApplyCuratedShort(cmd, "Releases", []string{"create", "view", "download"})

Some additional examples (as captured in the test cases):

1 command, all curated:

  pr:         list Pull Requests

2 commands, all curated:

  pr:         list and view Pull Requests

3 commands, all curated:

  pr:         list, view and create Pull Requests

4 commands, 3 curated:

  pr:         list, view and create Pull Requests (+1 more)

5 commands, 3 curated:

  pr:         list, view and create Pull Requests (+2 more)

2 commands, 2 curated, List capitalised:

  pr:         List and view Pull Requests

I added the curated list to each of the core commands, choosing the subcommands that best represent the breadth of functionality supported for that resource.

Also, if this PR does make it into consideration, worth noting that this is my first real world contribution in Go so I'd recommend a close review 🔎

Thanks!