uinput: support event types without codes

[Spaskich]

Summary

• Add a low-level enable_type() wrapper for UI_SET_EVBIT.
• Allow UInput capability mappings to enable an event type using an empty
  sequence of event codes.
• Document the behavior and add a regression test using EV_REP.

Why?

Some Linux input event types, notably EV_REP, need the event type itself to
be enabled but do not have a corresponding UI_SET_*BIT ioctl for individual
codes.

Currently, passing:

{
    ecodes.EV_KEY: [ecodes.KEY_A],
    ecodes.EV_REP: [],
}

does not enable EV_REP, because the empty list produces no calls to the
low-level uinput API.

This change treats an empty sequence as a request to enable the event type
without enabling any event codes. It allows uinput keyboard implementations to
opt into kernel-managed key repeat through the public UInput API.

Testing

• All tests passed.
• Created a real uinput keyboard with EV_REP: [].
• Verified that InputDevice.repeat reports non-zero delay and repeat values.
• Verified that held keys generate kernel repeat events and stop on release.