Use one implementation for finding path values

[chilagrow]

Description

Closes #2348.

I created a new package commonpath.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• 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 #3087 (307792e) into main (60ae858) will decrease coverage by 0.54%.
The diff coverage is 88.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3087      +/-   ##
==========================================
- Coverage   75.62%   75.08%   -0.54%     
==========================================
  Files         393      395       +2     
  Lines       21968    21953      -15     
==========================================
- Hits        16613    16484     -129     
- Misses       4390     4510     +120     
+ Partials      965      959       -6     

Files Changed	Coverage Δ
integration/compattestcaseresulttype_string.go	0.00% <ø> (ø)
integration/helpers.go	87.23% <ø> (+0.35%)	⬆️
internal/backends/error.go	54.28% <ø> (+2.85%)	⬆️
internal/backends/errorcode_string.go	50.00% <ø> (ø)
...ommon/aggregations/stages/projection/projection.go	86.97% <ø> (ø)
internal/handlers/common/sort.go	91.42% <ø> (ø)
internal/types/array.go	85.32% <ø> (ø)
internal/types/document.go	90.99% <ø> (ø)
internal/types/patherrorcode_string.go	0.00% <0.00%> (ø)
internal/handlers/pg/pgdb/query.go	81.96% <33.33%> (-0.66%)	⬇️
... and 18 more

... and 17 files with indirect coverage changes

Flag	Coverage Δ
hana	?
integration	71.40% <86.36%> (-0.63%)	⬇️
mongodb	5.36% <3.14%> (-0.01%)	⬇️
pg	65.05% <86.01%> (-0.16%)	⬇️
shard-1	56.29% <77.97%> (+0.98%)	⬆️
shard-2	52.60% <75.52%> (-0.51%)	⬇️
shard-3	55.52% <76.92%> (-2.87%)	⬇️
sqlite	33.61% <44.75%> (-4.82%)	⬇️
unit	24.97% <41.60%> (+0.27%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[AlekSi]

(please review, but don't merge yet)

[rumyantseva]

Based on my past experience with path-related code, the implementation looks clear to me. Asked a couple of questions where I'm not sure.

[noisersup]

Overall it seems that it handles existing logic and it's great that we merge all of those functions into the single one with options

[noisersup]

LGTM

[mergify]

@chilagrow this pull request has merge conflicts.

[rumyantseva]

In addition to the changes, I also like the documentation and examples (e.g. $unwind). It brings more clarity about how things work and why!