Optional sticky headers

[Earthsplit]

This PR adds an optional isSticky prop on EmojiPicker.Root to toggle the sticky behavior of category headers.

Closes #3

[marcbouchenoire]

Thanks for contributing!

I'd prefer to name the prop sticky to follow native props like disabled.
Could you also add tests in emoji-picker.test.browser.tsx to test the behaviors with sticky disabled?

[Earthsplit]

I've renamed the prop and added the test. Should be fine now

[marcbouchenoire]

Thanks! I'll release v0.3 with this change later this week.

[marcbouchenoire]

Done!