Add "checkup" or "doctor" option to `gp` command line

Add a command checkup to the gp tool that does some basic "diagnostics"/"troubleshooting"/"sanity checks" on the workspace/repo.

For example:

• Does it have a .gitpod.yml?
  • If not, does it have something else that might be a .gitpod.yml, e.g., gitpod.yml or .gitpod.yaml?
  • If so, does it syntax check? Any unknown attributes? Anything else that can be detected that is wrong about it?
    • E.g., it references an image: image doesn't exist. It references a custom dockerfile: that doesn't exist. It references a vscode extension: that extension doesn't exist at open-vsx.
    • E.g., it looks like it is trying to put something under /home/gitpod in an init: start task section (!! extra credit !!)
    • Are duplicate ports specified?
    • If there's an additionalRepositories key do the named repos exist?
    • If there's a mainConfiguration key does the named repo exist and have a .gitpod.yml pointing back to this repo?
• Does it have a custom dockerfile?
  • Does it syntax check?
  • Is there something that looks like it could be a custom dockerfile (.e.g, a file named .gitpod.Dockerfile per the documentation examples) but it isn't mentioned in the .gitpod.yml? (Or there isn't a .gitpod.yml?)
  • Does it look like the custom dockerfile is trying to put stuff under /workspace?
• Look in start task logs, the dotfile repo log, to find problems
  • task errored out on last run
  • task timed out on last run
• How big is the workspace relative to the resource limits (mainly, but probably not exclusively, max size of the backup archive)

Etc. etc. Goal is to help a gitpod user (especially, but not only, new users) quickly get on board and/or fix problems that are impacting them.

• (checkup is what it is called in the scoop command line tool, for example - probably others too.)
• (doctor is what it is called in the brew command line tool, for example.)

Start at the gp command-line entry point and map the proposed diagnostic areas before choosing a focused scope. Define which checks are included, then verify that the command reports actionable configuration, task-log, and workspace-size problems as described in the issue.