Add more validation and tests for $unset

[chilagrow]

Description

Closes #1432.

• Added validation of $unset input for invalid path.
• Moved conflict path detection logic to types package, previously it was in update.go.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2853 (660dd23) into main (2b077d5) will decrease coverage by 4.87%.
The diff coverage is 93.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2853      +/-   ##
==========================================
- Coverage   63.77%   58.91%   -4.87%     
==========================================
  Files         448      448              
  Lines       22890    22970      +80     
==========================================
- Hits        14598    13532    -1066     
- Misses       7334     8458    +1124     
- Partials      958      980      +22     

Impacted Files	Coverage Δ
internal/handlers/commonerrors/error.go	70.00% <ø> (-10.00%)	⬇️
internal/handlers/commonerrors/errorcode_string.go	80.00% <ø> (ø)
internal/types/documentpatherrorcode_string.go	0.00% <ø> (ø)
internal/handlers/common/update.go	79.95% <75.00%> (-9.75%)	⬇️
internal/types/path.go	95.07% <91.66%> (-0.35%)	⬇️
...ernal/handlers/common/aggregations/stages/unset.go	96.63% <96.29%> (+3.78%)	⬆️

... and 44 files with indirect coverage changes

Flag	Coverage Δ
integration	51.73% <93.16%> (-5.11%)	⬇️
mongodb	4.24% <0.00%> (-0.02%)	⬇️
pg	51.56% <93.16%> (-5.20%)	⬇️
shard-1	4.24% <0.00%> (-39.18%)	⬇️
shard-2	42.29% <89.74%> (+1.06%)	⬆️
shard-3	44.30% <56.41%> (+1.74%)	⬆️
unit	24.73% <0.00%> (-0.10%)	⬇️

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[mergify]

@chilagrow this pull request has merge conflicts.

[mergify]

@chilagrow this pull request has merge conflicts.

[noisersup]

LGTM

[chilagrow]

👋 @shibasisp, I added additional validation of $unset arguments in this PR 🤗

[rumyantseva]

LGTM