`CUSTOM_GETTERS_SETTERS` false positive when a property has a backing field

[diphtongue]

What's done:

• fixed warning CUSTOM_GETTERS_SETTERS on properties with backing field.
• added warning tests.

Closes #1709

[github-actions[bot]]

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

   164 files  ±0     164 suites  ±0   6m 33s :stopwatch: -55s 1 399 tests +2  1 382 :heavy_check_mark: +2  17 :zzz: ±0  0 :x: ±0  2 778 runs  +2  2 761 :heavy_check_mark: +2  17 :zzz: ±0  0 :x: ±0 

Results for commit d4d8feae. ± Comparison against base commit b5c1a374.

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

[github-actions[bot]]

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

   164 files  ±0     164 suites  ±0   9m 20s :stopwatch: +20s 1 399 tests +2  1 363 :heavy_check_mark: +2  36 :zzz: ±0  0 :x: ±0  2 778 runs  +2  2 742 :heavy_check_mark: +2  36 :zzz: ±0  0 :x: ±0 

Results for commit d4d8feae. ± Comparison against base commit b5c1a374.

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

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (b5c1a37) 78.62% compared to head (d4d8fea) 78.64%.

Files	Patch %	Lines
...m/saveourtool/diktat/ruleset/utils/AstNodeUtils.kt	68.18%	1 Missing and 6 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1815 +/- ## ============================================ + Coverage 78.62% 78.64% +0.02% + Complexity 2441 2432 -9 ============================================ Files 126 126 Lines 8555 8562 +7 Branches 2163 2171 +8 ============================================ + Hits 6726 6734 +8 - Misses 861 862 +1 + Partials 968 966 -2 ```

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