Description of change

In native text widgets, the text toolbar is shown when a user uses the pencil
to select text on an iPad running iOS 14 or higher.
To replicate this behavior, the platform sends a textInput channel message
called TextInputClient.showToolbar.
This notifies the Dart code that the toolbar should be shown.

When a user holds the pencil down, a visual gap in the text is shown to allow
the user extra space to write.
To replicate this behavior, the platform sends textInput channel messages
called TextInputClient.insertTextPlaceholder and
TextInputClient.removeTextPlaceholder.
Multiline text inputs should have placeholders that provide vertical space,
while single line inputs should provide horizontal space.

Migration guide

If you previously implemented TextEditingClient, you must override
showToolbar, insertTextPlaceholder, and removeTextPlaceholder to either
support these Scribble features or provide an empty implementation.