Update checkbox-v2.ts

[MaddipatlaChetan24]

Fixes #1, Fixes #2

Description

CheckboxConfig's picked changeHandlers list included onMouseDown
and the complete onMouseEnter/onMouseLeave pair, but omitted
onMouseUp. Confirmed against baseui's own API docs that Checkbox
supports onMouseUp as a first-class prop alongside onMouseDown,
onMouseEnter, onMouseLeave, onFocus, and onBlur — so this was
a real supported handler missing from the playground config, not a
deliberate omission of an unsupported prop.

Fix: added "onMouseUp" to the picked list. Assumes changeHandlers
in ./common/common already defines an onMouseUp entry parallel to
its existing onMouseDown entry — please confirm that file has it
before merging, since I don't have its contents.

Scope

Patch: Bug Fix