Add OPENSWIFTUI_ENABLE_PRIVATE_IMPORTS

[Kyle-Ye]

Add OPENSWIFTUI_ENABLE_PRIVATE_IMPORTS env to control the usage of @_private(sourceFile:) import

See more information here: https://github.com/swiftlang/swift/blob/main/docs/ReferenceGuides/UnderscoredAttributes.md#_privatesourcefile-filenameswift

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 24.31%. Comparing base (ee10119) to head (44253b7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #540      +/-   ##
==========================================
- Coverage   24.31%   24.31%   -0.01%     
==========================================
  Files         506      506              
  Lines       28979    28979              
==========================================
- Hits         7047     7046       -1     
- Misses      21932    21933       +1     

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