[native] [9/34] ReactNav6: separate StackNavigationProp from StackNavigationHelpers

Summary:
We want to type setParams correctly for both the specific-screen and any-screen-on-a-navigator case. As a result, the type of setParams is incompatible between the two.

Note that I have left off setParams from NavigationHelpers in this diff. That's because I need to make some changes to our codebase before I can do that without leading to Flow errors... otherwise the two setParams will collide. I will reintroduce setParams to NavigationHelpers in a later diff in the stack.

Depends on D5401

Test Plan: Flow + ran the app on my phone and played around with it

Reviewers: atul, tomek, bartek

Reviewed By: atul

Subscribers: abosh

Differential Revision: https://phab.comm.dev/D5402