Fixed warning `FUNCTION_BOOLEAN_PREFIX` on `operator fun`

[diphtongue]

What's done:

• fixed warning FUNCTION_BOOLEAN_PREFIX on operator fun.
• added warning test.

Closes #1723

[github-actions[bot]]

JUnit Tests (macOS, EnricoMi/publish-unit-test-result-action@v2)

   164 files  ±0     164 suites  ±0   10m 35s :stopwatch: + 6m 46s 1 388 tests +1  1 345 :heavy_check_mark: +1  43 :zzz: ±0  0 :x: ±0  2 767 runs  +1  2 724 :heavy_check_mark: +1  43 :zzz: ±0  0 :x: ±0 

Results for commit 6fa6ade4. ± Comparison against base commit c28f1a1c.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

JUnit Tests (Windows, EnricoMi/publish-unit-test-result-action@v2)

   164 files  ±0     164 suites  ±0   8m 19s :stopwatch: + 2m 29s 1 388 tests +1  1 357 :heavy_check_mark: +1  31 :zzz: ±0  0 :x: ±0  2 767 runs  +1  2 736 :heavy_check_mark: +1  31 :zzz: ±0  0 :x: ±0 

Results for commit 6fa6ade4. ± Comparison against base commit c28f1a1c.

:recycle: This comment has been updated with latest results.

[codecov[bot]]

Codecov Report

Merging #1767 (6fa6ade) into master (c28f1a1) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1767   +/-   ##
=========================================
  Coverage     78.25%   78.25%           
+ Complexity     2410     2409    -1     
=========================================
  Files           126      126           
  Lines          8451     8453    +2     
  Branches       2148     2149    +1     
=========================================
+ Hits           6613     6615    +2     
  Misses          880      880           
  Partials        958      958           

Files	Coverage Δ
.../diktat/ruleset/rules/chapter1/IdentifierNaming.kt	82.72% <100.00%> (-0.16%)	:arrow_down:
...m/saveourtool/diktat/ruleset/utils/AstNodeUtils.kt	80.62% <100.00%> (+0.20%)	:arrow_up: