Add parsing playground

- `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.