feat: added customizeable close button

[ljukas]

Summary

Adds basic functionality for customising close button.
Moved the rendering to a useMemo hook to make it easier to follow the logic flow, since I did not want to break existing usages of the closeButton prop

Test plan

There is an example usage in the example app. Basically it tries to mimic the action and cancel actions except we only accept a ReactNode instead of a ToastAction

Fixes #129

[gunnartorfis]

👏 just a minor comment about typechecking

[ljukas]

Just a note, I think a continuation of the custom-component structure could be to accept a function instead and there we can pass theme prop etc for the custom components and they can depend on that. Makes it easier to wrap a wrapped around sonner-native for custom styles etc

[gunnartorfis]

Released in 0.15.0, thanks for the contribution! 👏