Implement tapegen doctor

[MarcelGarus]

• show the OS and version
• show the Dart version
• show the Flutter version, if applicable irrelevant
• show the tape version
• show the tapegen version
• show the versions of all used taped-packages
• show a list of all TapeAdapters defined in the project
  • show their name
  • show the file path
  • show their registration status (including id)
  • show the tree hierarchy
• show a GitHub link if it's a (public?) Git repo
• show link to issue page with template selected
  Irrelevant in tapegen doctor itself. This should happen somewhere else, where an error actually occurs. Here, we don't have enough information to decide on an appropriate issue template.