feat: add prefer-immediate-return rule

[roman-petrov]

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

[ ] Documentation update
[ ] Bug fix
[x] New rule
[ ] 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)

#474

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

The fixer implemented is something like "semi-automatic" fixer, it does only half of the work required :) It replaces only expression in the problematic return statement and code contains unused variable after that.

For example,

int calculateSum(int a, int b) {
   final sum = a + b; 
   return sum;
}

will be fixed as

int calculateSum(int a, int b) {
   final sum = a + b; 
   return a + b;
}

and sum variable should be manually removed. I think that this shoud be acceptable - it's not a huge amount of work to manually remove unused variable (linters should warn about it too).

Implementing fully automatic fixer seems to be a bit hard: we have to replace multiple nodes/statements, but currently (as I see) we have only nodeLocation utility method which can create location only from single node.

[codecov]

Codecov Report

Merging #736 (ef802a8) into master (f290337) will increase coverage by 0.03%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #736      +/-   ##
==========================================
+ Coverage   86.84%   86.87%   +0.03%     
==========================================
  Files         269      271       +2     
  Lines        5730     5762      +32     
==========================================
+ Hits         4976     5006      +30     
- Misses        754      756       +2     

Impacted Files	Coverage Δ
...c/analyzers/lint_analyzer/rules/rules_factory.dart	90.36% <0.00%> (-2.24%)	⬇️
...immediate_return/prefer_immediate_return_rule.dart	100.00% <100.00%> (ø)
...es/rules_list/prefer_immediate_return/visitor.dart	100.00% <100.00%> (ø)

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[incendial]

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