[ScriptUI] onClose _can_ return false to cancel, but it is not required

[fartinmartin]

The current definition of the onClosed callback says:

...The function is called before the window actually closes; it can return false to cancel the close operation.

But, the type suggests a boolean must be returned:

Types-for-Adobe/shared/ScriptUI.d.ts

Line 349 in e84efaf

onClose(): boolean

This PR updates the return type to void | boolean.

[zlovatt]

Thank you!