fix: cover more cases in "prefer-immediate-return" rule

[roman-petrov]

Fixes #754

I just added more test cases and visitor now overrides visitBlock instead of overriding visitBlockFunctionBody to cover these cases in prefer-immediate-return rule.

[codecov]

Codecov Report

Merging #760 (a9d02dc) into master (76705e5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #760   +/-   ##
=======================================
  Coverage   86.99%   86.99%           
=======================================
  Files         273      273           
  Lines        5784     5784           
=======================================
  Hits         5032     5032           
  Misses        752      752           

Impacted Files	Coverage Δ
...es/rules_list/prefer_immediate_return/visitor.dart	100.00% <100.00%> (ø)

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[roman-petrov]

"Auto Author Assign" check failed - seems this is not related to my PR so it should be ready for review.