fix: skip synthetic tokens while compute 'Source lines of Code'

[dkrutskikh]

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

[ ] Documentation update
[X] 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:

What changes did you make? (Give an overview)

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

[codecov]

Codecov Report

Merging #487 (06bd4b4) into master (eb5412c) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
- Coverage   83.73%   83.62%   -0.12%     
==========================================
  Files         199      199              
  Lines        4114     4110       -4     
==========================================
- Hits         3445     3437       -8     
- Misses        669      673       +4     

Impacted Files	Coverage Δ
...alyzers/lint_analyzer/metrics/metrics_factory.dart	100.00% <100.00%> (ø)
...list/source_lines_of_code/source_code_visitor.dart	100.00% <100.00%> (ø)
lib/src/config_builder/config_builder.dart	81.81% <100.00%> (-1.52%)	⬇️
...tterns/anti_patterns_list/long_parameter_list.dart	95.65% <0.00%> (-4.35%)	⬇️
.../anti_patterns/anti_patterns_list/long_method.dart	96.36% <0.00%> (-3.64%)	⬇️

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