Add parsing playground

[royalicing]

• GET: https://regenerated.dev/parse
  • textbox("Parser source")
  • textbox("Example input")
  • button("Go")

Enter example input and source to a parse, and see the results. Allows fast iteration all from a browser. Then once done, copy-and-paste to a real project.

Provide examples of different parsers, such as datetime, math, and email address.