[SuperEditor] Fix immutability violation in spellchecking styler (Resolves #2560)

[angelosilvestre]

[SuperEditor] Fix immutability violation in spellchecking styler (Resolves #2560)

A crash was reported within spelling_and_grammar.dart within _applyErrors() where we try to do the following:

viewModel.grammarErrors
  ..clear()
  ..addAll([
    //....
  ]);

I didn't find any places where we return an unmodifiable list from the the grammarErrors, but I think it's safer to replace the list instead of modifying it, because apps can add custom view models, that might return an unmodifiable list for grammarErrors.

This PR changes the SpellingAndGrammarStyler to override the viewmodel's spellingErrors and grammarErrors.

[matthew-carroll]

LGTM