Fixes # 3966. TextField crashes app when pasting unicode surrogate pair

[BDisp]

Fixes

• Fixes TextField crashes app when pasting unicode surrogate pair #3966

Proposed Changes/Todos

• Add Key(int) constructor
• Add implicit operator Key(int value)
• Allow Key handle non-BMP code points
• Fix TextField non-BMP issues
• Fix TextField PosisionCursor wide chars issue

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

As part of this PR, would you please move any parallelizable unit tests for TextField to the parallelizable unit tests?

[BDisp]

You're asking to move to the same unit test type 😃

[BDisp]

Sorry I misunderstand. Can you help me to identify how I can check if an unit test can be parallelizable, please? Thanks.

[tig]

If it doesn't use AutoInitShutdown, SetupFakeDriver, Application, MouseGrab, or CM it can probably be moved.