Add backend capability query function

Users can't discover at runtime which features their backend supports.
For example, pntr has no axes, raylib has no L2/R2 digital, only SDL3
provides per-controller button names. The only way to find out is trial
and error or reading the source.

Suggested Implementation

Add nk_gamepad_input_source_supports() or a capabilities bitmask in
struct nk_gamepad_input_source:

• NK_GAMEPAD_CAP_AXES
• NK_GAMEPAD_CAP_HOTPLUG
• NK_GAMEPAD_CAP_BUTTON_NAMES
• NK_GAMEPAD_CAP_RUMBLE (future)

Each backend sets its capabilities during init.

QA

• nk_gamepad_input_source_supports(gp, NK_GAMEPAD_CAP_AXES) returns false for pntr
• returns true for SDL/SDL3/GLFW/raylib