Simplify word expansion functions

[carlos-zamora]

Summary of the Pull Request

Simplifies the word expansion functions in TextBuffer by removing the following functions:

• GetWordStart2()
• GetWordEnd2()
• MoveToNextWord()
• MoveToPreviousWord()
• _GetWordStartForAccessibility()
• _GetWordStartForSelection()
• _GetWordEndForAccessibility()
• _GetWordEndForSelection()

In favor of a simple:

• GetWordStart()
• GetWordEnd()
• _GetDelimiterClassRunStart()`
• _GetDelimiterClassRunEnd()

Tests were used to help ensure a regression doesn't occur. That said, there were a few modifications there too:

• Removed MoveByWord test
  • It directly called MoveToNextWord() and MoveToPreviousWord(), which no longer exist. These were helper functions for UiaTextRangeBase, and any special logic has been moved into _moveEndpointByUnitWord using the unified GetWordStart()/GetWordEnd() APIs.
  • The tested behavior is still covered by MoveToPreviousWord, MovementAtExclusiveEnd, and the generated word movement tests.
• updated GetWordBoundaries tests
  • Inclusive --> exclusive end positions for GetWordEnd(): The old _GetWordEndForSelection() returned inclusive end positions, whereas the new GetWordEnd() returns exclusive end positions. This is what GetWordEnd2() already used, so every old expected value shifted +1 in the x direction (or to {RightExclusive, y} at row boundaries) to account for that.
  • ControlChar wrap-crossing behavior: The old _GetWordStartForSelection() had a special check at the left boundary that prevented whitespace runs from crossing wrapped row boundaries. The new _GetDelimiterClassRunStart() doesn't have this special case (it treats ControlChar the same as other delimiter classes when the row was wrap-forced). This changed one test case: GetWordStart({1, 4}) in selection mode went from {0, 4} to {6, 3} (the whitespace run now crosses the wrap boundary to row 3). This matches the behavior TerminalSelection was already getting from GetWordStart2().

Validation Steps Performed

Tests passed:
✅ Conhost.Interactivity.Win32.Unit.Tests.dll
✅ UnitTests_TerminalCore\Terminal.Core.Unit.Tests.dll

Word navigation feels good for...
✅ Narrator
✅ NVDA
✅ Mouse selection
✅ Mark mode

Closes #4423

[lhecker]

So, in essence, this is "if false, add CRLF/newlines to the wordDelemiters list", right?

[DHowett]

I think we need to test conhost, expressly, with this change. I'm afraid of the test values changing--because my mental model is that a refactor shouldn't also require us to change the tests (how would we ensure the refactor was correct if the original tests didn't pass?)