fix: setting headerBackTitleVisible to false not working on iOS

[zetavg]

Motivation

According to the docs, setting headerBackTitleVisible to false should hide the back title on iOS, while this is not the case with react-native-screens v3.29+.

Cause

With the current code, backTitle={headerBackTitleVisible ? headerBackTitle : ' '}, the back title is set to a blank string if headerBackTitleVisible is false. However, in newer versions of react-native-screens, the backTitle will be considered blank if it only contains white spaces, making it fallback to the default title - which makes the back title not hidden at all.

Fix

Assign the backTitleVisible property of RNSScreenStackHeaderConfig native component, which seems to be a legit way to hide the back title.

Test plan

1. Create a native stack navigator with the backTitleVisible screen option set to false.
2. See if the back title is hidden on iOS when navigating to another screen.

Expected	Not Expected

• [x] May need to test if assigning the new property will break older versions of react-native-screens.
  • Tested with react-native-screens 3.20.0 and 3.21.0, app does not crash and the back title remains hidden-able.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 77.00%. Comparing base (d90ed76) to head (84cca6c). Report is 2 commits behind head on main.

Files	Patch %	Lines
packages/native-stack/src/views/HeaderConfig.tsx	33.33%	1 Missing and 1 partial :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11937 +/- ## ========================================== - Coverage 77.03% 77.00% -0.03% ========================================== Files 210 210 Lines 6339 6341 +2 Branches 2506 2508 +2 ========================================== Hits 4883 4883 - Misses 1402 1403 +1 - Partials 54 55 +1 ```

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