Add `drop` to `Text.Megaparsec`

[sol]

I'd love to have Text.Megaparsec to export

-- | Advance by / ignore `n` input tokens
drop :: MonadParsec e s m => Int -> m ()
drop n = P.skipCount n $ P.token Just mempty

(or possibly implement it with getOffset / setOffset, if that results in the exact same parser state, not sure)