feat: remove declaration in prefer-immediate-return

[ookami-kb]

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

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

What changes did you make? (Give an overview)

prefer-immediate-return will remove variable declaration as well, if this is a single declaration (e.g. it will remove final x = 1 but not final a = 2, x = 1).

It will solve the most common use case, when there's a single variable declaration and immediate return.

Is there anything you'd like reviewers to focus on?

[codecov]

Codecov Report

Merging #824 (78f688e) into master (43a293a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #824   +/-   ##
=======================================
  Coverage   87.26%   87.27%           
=======================================
  Files         277      277           
  Lines        5898     5902    +4     
=======================================
+ Hits         5147     5151    +4     
  Misses        751      751           

Impacted Files	Coverage Δ
...es/rules_list/prefer_immediate_return/visitor.dart	100.00% <ø> (ø)
...immediate_return/prefer_immediate_return_rule.dart	100.00% <100.00%> (ø)
lib/src/utils/node_utils.dart	100.00% <100.00%> (ø)

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