Move cursor to edge of selection when moving caret left/right

This is to mimic the behavior of many third party text editors.
The reasons it's not doing it when moving by word is due to that
behavior being mostly the same on other editors.

This was backported to 3.x from pull request #51502.

scene/gui/text_edit.cpp

@@ -2812,7 +2812,7 @@ void TextEdit::_gui_input(const Ref<InputEvent> &p_gui_input) {
 
 		_reset_caret_blink_timer();
 
-		// Save here for insert mode, just in case it is cleared in the following section.
+		// Save here for insert mode as well as arrow navigation, just in case it is cleared in the following section.
 		bool had_selection = selection.active;
 
 		// Stuff to do when selection is active.
@@ -3168,6 +3168,11 @@ void TextEdit::_gui_input(const Ref<InputEvent> &p_gui_input) {
 			case KEY_LEFT: {
 				if (k->get_shift()) {
 					_pre_shift_selection();
+				} else if (had_selection && !k->get_command() && !k->get_alt()) {
+					cursor_set_line(selection.from_line);
+					cursor_set_column(selection.from_column);
+					deselect();
+					break;
 #ifdef APPLE_STYLE_KEYS
 				} else {
 #else
@@ -3245,6 +3250,11 @@ void TextEdit::_gui_input(const Ref<InputEvent> &p_gui_input) {
 			case KEY_RIGHT: {
 				if (k->get_shift()) {
 					_pre_shift_selection();
+				} else if (had_selection && !k->get_command() && !k->get_alt()) {
+					cursor_set_line(selection.to_line);
+					cursor_set_column(selection.to_column);
+					deselect();
+					break;
 #ifdef APPLE_STYLE_KEYS
 				} else {
 #else