Make _handle param f positional-only to avoid form field collision

[lisadlima]

Closes #834

• f as a parameter triggers TypeError: _handle() got multiple values for argument 'f', because the form field f arrives via **wreq and collides with _handle's positional f (the handler itself).

• Makes f positional-only with /: async def _handle(f, /, *args, **kwargs). This is the standard stdlib idiom (functools.partial, functools.reduce) and is preferable to renaming (e.g. _f), which only relocates the collision to a different field name.

• notebook fasthtml/nbs/api/00_core.py was edited, synced (not core.py directly), cleaned. Test is in nbs/api/00_core.ipynb right under _handle.