Add headless spinbox

[agluszak]

Summary

• Add generic headless SpinBox direction intent and increment/decrement button markers
• Register and export SpinBox from bevy_ui_widgets
• Add the spinbox_months example as a non-numeric enum use case

Validation

• cargo test -p bevy_ui_widgets spinbox
• cargo check -p bevy --example spinbox_months

[alice-i-cecile]

Related to #24060.