Make it easier to see if a `Task` is created/running/exited

It is currently possible to check the status of a task with https://docs.python.org/3/library/inspect.html#inspect.getcoroutinestate, but this is fairly obscure and not very readable.
We should either document this, or add properties to Task that are easy to interface with.
For implementing it, I think has_started() -> bool and has_exited -> bool would be a clean way of doing it.

I personally encountered this in https://github.com/python-trio/trio/issues/3035#issuecomment-2312328838 which would only be for internal use, but I'm guessing it might be useful enough to surface in the public API.

See discussion in Gitter: https://matrix.to/#/!OqDVTrmPstKzivLwZW:gitter.im/$umF5oOw9d2Z20gb95GF_C48HyBi2l6eD3ZW4r_4ark0?via=gitter.im&via=matrix.org&via=zoltan.site

Start by reviewing Python's inspect.getcoroutinestate documentation and the Task API, then read the linked Trio discussion and Gitter conversation for the intended public behavior. Decide whether documentation alone is sufficient or whether Task status properties are needed; done means the chosen approach clearly distinguishes created, running, and exited states.