Support tests as comments to the .pest files, like Rust's doctest

[nyurik]

I love Rust's documentation tests, and I wonder if it would be possible to support a similar concept in the .pest files. The actual syntax is TBD.

/// Integer numbers (positive and negative)
/// pass: `0`, `42`, `+42`, `-42`
/// fail: `abc`, `+`, `--`
integer = @{ ("+" | "-")? ~ ASCII_DIGIT+ }