Make nk_button_behavior public

This function is pretty useful, i think it should be available in public API.

Locate nk_button_behavior in Nuklear's single-header library and inspect how its public API declarations are organized. Compare it with the other button-related entry points, then use the project's existing build or examples to verify external access. Done means the function is consistently exposed through the public API without breaking the library's existing interface.

Written by the indexing model from the issue text.