It's now possible to use the native-rendered text selection context menu on iOS. This sacrifices customizability in exchange for avoiding showing a notification when the user presses "Paste". It's off by default, but to enable, see the example system_context_menu.0.dart.

Reverts: #148238
Initiated by: zanderso
Reason for reverting: Failures in post submit https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8748025189669617649/+/u/run_test.dart_for_web_canvaskit_tests_shard_and_subshard_3/stdout
Original PR Author: justinmc
Reviewed By: {hellohuanlin}
This change reverts the following previous change:
Reland of #143002, which was reverted in #148237 due to unresolved docs references. Not sure why those weren't caught in presubmit.
```
dartdoc:stdout: Generating docs for package flutter...
dartdoc:stderr:   error: unresolved doc reference [TextInput.showSystemContextMenu]
dartdoc:stderr:     from widgets.MediaQueryData.supportsShowingSystemContextMenu: (file:///b/s/w/ir/x/w/flutter/packages/flutter/lib/src/widgets/media_query.dart:579:14)
dartdoc:stderr:     in documentation inherited from widgets.MediaQueryData.supportsShowingSystemContextMenu: (file:///b/s/w/ir/x/w/flutter/packages/flutter/lib/src/widgets/media_query.dart:579:14)
dartdoc:stderr:   error: unresolved doc reference [showSystemContextMenu]
dartdoc:stderr:     from services.SystemContextMenuController.hide: (file:///b/s/w/ir/x/w/flutter/packages/flutter/lib/src/services/text_input.dart:2554:16)
dartdoc:stderr:   error: unresolved doc reference [hideSystemContextMenu]
dartdoc:stderr:     from services.SystemContextMenuController.show: (file:///b/s/w/ir/x/w/flutter/packages/flutter/lib/src/services/text_input.dart:2509:16)
```

Reverting #148238 which was relanded in #148237 which was originally from #143002.
This time post-submit I saw a failure of a wasm version of a test added by this PR.