feat: add allow-nullable config option for avoid-returning-widgets

[incendial]

Please write a short comment explaining your change (or "none" for internal only changes)

New option for avoid-returning-widgets rule.

Rule documentation

Use allow-nullable configuration, if you want to allow functions or methods that return nullable Widget.

[codecov]

Codecov Report

Merging #1180 (06e927e) into master (cde4648) will increase coverage by 0.00%.
The diff coverage is 97.43%.

@@           Coverage Diff           @@
##           master    #1180   +/-   ##
=======================================
  Coverage   86.11%   86.12%           
=======================================
  Files         359      359           
  Lines        8075     8085   +10     
=======================================
+ Hits         6954     6963    +9     
- Misses       1121     1122    +1     

Impacted Files	Coverage Δ
..._collection_methods_with_unrelated_types_rule.dart	100.00% <ø> (ø)
...rtions/avoid_unnecessary_type_assertions_rule.dart	100.00% <ø> (ø)
..._type_casts/avoid_unnecessary_type_casts_rule.dart	100.00% <ø> (ø)
...les_list/member_ordering/member_ordering_rule.dart	80.95% <ø> (ø)
...teral_compare/no_boolean_literal_compare_rule.dart	100.00% <ø> (ø)
...st/prefer_iterable_of/prefer_iterable_of_rule.dart	100.00% <ø> (ø)
...eturning_widgets/avoid_returning_widgets_rule.dart	75.00% <66.66%> (-2.28%)	⬇️
...llection_methods_with_unrelated_types/visitor.dart	100.00% <100.00%> (ø)
...id_redundant_async/avoid_redundant_async_rule.dart	100.00% <100.00%> (ø)
...ules/rules_list/avoid_redundant_async/visitor.dart	100.00% <100.00%> (ø)
... and 11 more

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