Don't scroll vertically on horizontal scroll motions (#10979)

## Summary of the Pull Request

Pretty straightforward. Check if the scroll event is a horizontal movement. If it is, ignore it. We don't have a horizontal scrollbar.

## References
* obviously, revisit this if we ever do #1860 

## PR Checklist
* [x] Closes #10329
* [x] I work here
* [ ] Tests added/passed
* [n/a] Requires documentation to be updated

## Validation Steps Performed
* scrolled ↑/↓ with slaptop trackpad: terminal scrolls.
* scrolled ←/→ with slaptop trackpad: terminal doesn't scroll.
* Scrolling _slightly more vertically than horizontally_ still scrolls.
* Scrolling _slightly more horizontally than vertically_ doesn't scroll.

src/cascadia/TerminalControl/TermControl.cpp

@@ -1223,6 +1223,12 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         _RestorePointerCursorHandlers(*this, nullptr);
 
         const auto point = args.GetCurrentPoint(*this);
+        // GH#10329 - we don't need to handle horizontal scrolls. Only vertical ones.
+        // So filter out the horizontal ones.
+        if (point.Properties().IsHorizontalMouseWheel())
+        {
+            return;
+        }
 
         auto result = _interactivity.MouseWheel(ControlKeyStates{ args.KeyModifiers() },
                                                 point.Properties().MouseWheelDelta(),