feat(collections): add index arg to methods

[canac]

Adds an index arg to collections' predicate/selector/transformer/discriminator method. I added it only to collection helpers that operate on iterables because collection helpers that operate on records don't seem as likely to need access to the key index. Please let me know if I missed any.

Closes #6288

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.23%. Comparing base (3b75ee7) to head (bff3bbf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6382      +/-   ##
==========================================
- Coverage   96.23%   96.23%   -0.01%     
==========================================
  Files         556      556              
  Lines       42065    42075      +10     
  Branches     6371     6371              
==========================================
+ Hits        40481    40490       +9     
- Misses       1544     1545       +1     
  Partials       40       40              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kt3k]

I'm generally in favor of the idea. It looks useful in several places, and also this is aligned to the design of builtin map methods (such as map, filter, forEach, etc)

What do people think?

---

Note: Currently we have policy of accepting any new feature in unstable_ prefixed path. That requires all these APIs duplicated in unstable_foo.ts, but that feels too unwieldy in this case. I'll discuss with core team about how to handle this particular case.