Add command key bindings to macOS text editing and fix selection. (#44130)

This adds support for the command key for text selection/editing on macOS. I had ported the text editing code (in #42879), but forgot to add support for the command key itself. This also adds a test that tests the text editing on multiple platforms instead of just testing Android.

There appears to still be a bug (filed #44135) where we're losing key events sometimes on macOS, leaving some keys "stuck" on, but this PR at least allows the right key combinations to be used.

packages/flutter/lib/src/painting/text_painter.dart

@@ -816,15 +816,15 @@ class TextPainter {
   /// <http://www.unicode.org/reports/tr29/#Word_Boundaries>.
   TextRange getWordBoundary(TextPosition position) {
     assert(!_needsLayout);
-    // TODO(gspencergoog): remove the List<int>-based code when the engine API
-    // returns a TextRange instead of a List<int>.
-    final dynamic boundary = _paragraph.getWordBoundary(position);
-    if (boundary is List<int>) {
-      final List<int> indices = boundary;
-      return TextRange(start: indices[0], end: indices[1]);
-    }
-    final TextRange range = boundary;
-    return range;
+    return _paragraph.getWordBoundary(position);
+  }
+
+  /// Returns the text range of the line at the given offset.
+  ///
+  /// The newline, if any, is included in the range.
+  TextRange getLineBoundary(TextPosition position) {
+    assert(!_needsLayout);
+    return _paragraph.getLineBoundary(position);
   }
 
   /// Returns the full list of [LineMetrics] that describe in detail the various