Handle backspace in text fields (#68812)

Currently the framework handles delete, but not backspace, so embeddings
all have to implement backspace handling themselves. This eliminates
that inconsistency and allows simplified code in embeddings by adding
backspace handling.

It also fixes a bug uncovered in the delete handling where deleting a
selection would also delete the next character after the selection.

packages/flutter/lib/src/rendering/editable.dart

@@ -484,6 +484,7 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
     LogicalKeyboardKey.keyV,
     LogicalKeyboardKey.keyX,
     LogicalKeyboardKey.delete,
+    LogicalKeyboardKey.backspace,
   };
 
   static final Set<LogicalKeyboardKey> _nonModifierKeys = <LogicalKeyboardKey>{
@@ -544,7 +545,9 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
       // as the _handleKeyEvent method
       _handleShortcuts(key);
     } else if (key == LogicalKeyboardKey.delete) {
-      _handleDelete();
+      _handleDelete(forward: true);
+    } else if (key == LogicalKeyboardKey.backspace) {
+      _handleDelete(forward: false);
     }
   }
 
@@ -813,22 +816,27 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
     }
   }
 
-  void _handleDelete() {
+  void _handleDelete({ required bool forward }) {
     assert(_selection != null);
-    final String textAfter = selection!.textAfter(_plainText);
-    if (textAfter.isNotEmpty) {
-      final int deleteCount = nextCharacter(0, textAfter);
-      textSelectionDelegate.textEditingValue = TextEditingValue(
-        text: selection!.textBefore(_plainText)
-          + selection!.textAfter(_plainText).substring(deleteCount),
-        selection: TextSelection.collapsed(offset: selection!.start),
-      );
-    } else {
-      textSelectionDelegate.textEditingValue = TextEditingValue(
-        text: selection!.textBefore(_plainText),
-        selection: TextSelection.collapsed(offset: selection!.start),
-      );
+    String textBefore = selection!.textBefore(_plainText);
+    String textAfter = selection!.textAfter(_plainText);
+    int cursorPosition = selection!.start;
+    // If not deleting a selection, delete the next/previous character.
+    if (selection!.isCollapsed) {
+      if (!forward && textBefore.isNotEmpty) {
+        final int characterBoundary = previousCharacter(textBefore.length, textBefore);
+        textBefore = textBefore.substring(0, characterBoundary);
+        cursorPosition = characterBoundary;
+      }
+      if (forward && textAfter.isNotEmpty) {
+        final int deleteCount = nextCharacter(0, textAfter);
+        textAfter = textAfter.substring(deleteCount);
+      }
     }
+    textSelectionDelegate.textEditingValue = TextEditingValue(
+      text: textBefore + textAfter,
+      selection: TextSelection.collapsed(offset: cursorPosition),
+    );
   }
 
   /// Marks the render object as needing to be laid out again and have its text