Update textile parser for help files to new method

[jools-r]

The plugin template still refers to the now defunct classTextile.php. jsoo has a PR but I believe that is also for an earlier iteration of the new class-based method.

[Bloke]

Thank you! That patch looks pretty reasonable so I might steal the best bits and just commit them. It's not mergable as-is because I'm so lazy I didn't even see it had a PR raised and things have moved on. Leave it with me.