wxRichTextCtrl: Enhance macOS key bindings

Update key bindings for macOS to more closely match native behavior.

Currently, Cmd+Left goes to the left word boundary, and Cmd+Right goes
to the right word boundary, like on Windows (wxWidgets treats Cmd on Mac
as if it were Ctrl for keyboard modifier key purposes).  Alt/Opt+Left
and Alt/Opt+Right do nothing.

This change maps Cmd+Left to the start of the line, and Cmd+Right to the
end of the line.  It also maps Alt/Opt+Left to WordLeft() and
Alt/Opt+Right to WordRight(), which is conventional on native Cocoa
controls (including other wxWidgets controls that map directly to Cocoa
controls).

See #26064, #26066.

(cherry picked from commit 2048a6871c)

docs/changes.txt

@@ -259,6 +259,7 @@ wxMSW:
 wxOSX:
 
 - Fix wxChoice default size under macOS 26 Tahoe (#26058).
+- Improve wxRichTextCtrl handling of cursor keys (Lee Ockert, #26064).
 
 
 3.2.9: (released 2025-12-12)