Add underscores analyzer

[k1LoW]

ref: https://google.github.io/styleguide/go/decisions#underscores

[github-actions]

Code Metrics Report

	main (70a79e1)	#22 (03ccfd5)	+/-
Coverage	83.8%	82.6%	-1.2%
Code to Test Ratio	1:0.1	1:0.1	-0.0
Test Execution Time	4s	4s	0s

Details

  |                     | main (70a79e1) | #22 (03ccfd5) |  +/-  |
  |---------------------|----------------|---------------|-------|
- | Coverage            |          83.8% |         82.6% | -1.2% |
  |   Files             |              5 |             6 |    +1 |
  |   Lines             |            167 |           230 |   +63 |
+ |   Covered           |            140 |           190 |   +50 |
- | Code to Test Ratio  |          1:0.1 |         1:0.1 |  -0.0 |
  |   Code              |            700 |           860 |  +160 |
+ |   Test              |             85 |           103 |   +18 |
  | Test Execution Time |             4s |            4s |    0s |

Code coverage of files in pull request scope (93.8% → 82.3%)

Files	Coverage	+/-
analyzer/decisions/underscores/underscores.go	80.6%	+80.6%
detector/detector.go	88.2%	-5.5%

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Reported by octocov