Support fixes to create stateful and stateless widgets

[bwilkerson]

When an undefined name appears as a class name in a constructor invocation and the context type is Flutter's Widget (such as nested inside another constructor invocation or in a return statement), there should be quick fixes to 'Create stateful widget' and 'Create stateless widget'.

The fixes should create code similar to the corresponding snippets, but tailored by any additional information in the constructor invocation, such as a constructor name or parameter names and types.

[FMorschel]

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