bug fixed: Transition.downToUp have a black background container #1560

[Leobuaa]

• I add a parameter oldPage on api GetNavigationExt.to(), oldPage is the previous page when you push a new page, if you use a custom transition, we need to change the old page exit transition, otherwise it will show black container in some cases, it fixes the bug:
  transition: Transition.downToUp have a black background container #1560

• When we use Get.to(() => NextPage(), transition: Transition.downToUp), we should change the previous page's exit transition to SlideTopTransition, now use new api: Get.to(() => NextPage(), transition: Transition.downToUp, oldPage: currentPage)

before:

before.mp4

after:

after.mp4

Every PR must update the corresponding documentation in the code, and also the readme in english with the following changes.

Pre-launch Checklist

• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or @jonataslaw said the PR is test-exempt.
• All existing and new tests are passing.

[Leobuaa]

@jonataslaw

[jonataslaw]

Thank you very much for this PR!
I'm just wondering if there's a way to do this dynamically, without having to pass a page by parameter.

In fact, I think this problem only occurs on iOS, due to the cupertino animation, but this can be disabled.

If it only happens on iOS, we could disable the previous page animation when the animation is vertical.