Ruby: improve mixin detection

[masatake]

• detect method style mixin as suggested in Ruby: extend mixin field #2481, and
• ignore unnamed blocks like "while", "if", ... when detecting mixing method invocations.

[coveralls]

Coverage increased (+0.004%) to 86.714% when pulling ea87271 on masatake:ruby-improve-mixin-detection into 513872a on universal-ctags:master.

[codecov]

Codecov Report

Merging #2483 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2483   +/-   ##
=======================================
  Coverage   86.60%   86.61%           
=======================================
  Files         177      177           
  Lines       36763    36774   +11     
=======================================
+ Hits        31839    31850   +11     
  Misses       4924     4924           

Impacted Files	Coverage Δ
main/nestlevel.c	87.50% <100.00%> (-0.26%)	⬇️
parsers/ruby.c	98.12% <100.00%> (+0.08%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 999a44a...ea87271. Read the comment docs.