[SuperEditor] Fix missing default task component builder (Resolves #2048)

[angelosilvestre]

[SuperEditor] Fix missing default task component builder. Resolves #2048

SuperEditor crashes with the default configuration if the content contains a task:

======== Exception caught by widgets library =======================================================
The following _Exception was thrown building _StandardEditor(state: _StandardEditorState#cb5c0):
Exception: Couldn't find styler to create component for document node: TaskNode

The reason is that we don't add the task component builder by default.

This PR adds a TaskComponentBuilder when the app uses the default component builders.

Additionally, we have an UnknownComponentBuilder to render unknown components, but it only creates the component, not the view model. As a result, any unknown node causes the editor to crash. I think that it's better to display a placeholder instead of crashing the editor if we find an unknown node. I modified the UnknownComponentBuilder to return a new view model instead of returning null. Since the UnknownComponentBuilder is always the last, this change shouldn't cause any issues, but I can remove this change if it's not desired.

[matthew-carroll]

LGTM