feat: add allow-initialized option to avoid-late-keyword

[incendial]

What is the purpose of this pull request? (put an "X" next to an item)

• Documentation update
• Bug fix
• New rule
• Changes an existing rule
• Add autofixing to a rule
• Add a CLI option
• Add something to the core
• Other, please explain:

#1113

[codecov]

Codecov Report

Merging #1145 (3130ac9) into master (29e6181) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1145   +/-   ##
=======================================
  Coverage   85.54%   85.54%           
=======================================
  Files         349      350    +1     
  Lines        7870     7875    +5     
=======================================
+ Hits         6732     6737    +5     
  Misses       1138     1138           

Impacted Files	Coverage Δ
...st/avoid_late_keyword/avoid_late_keyword_rule.dart	100.00% <100.00%> (ø)
...s/rules_list/avoid_late_keyword/config_parser.dart	100.00% <100.00%> (ø)
...r/rules/rules_list/avoid_late_keyword/visitor.dart	100.00% <100.00%> (ø)

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