feat(collections): stabilize index argument in iterable methods

[tomas-zijdemans]

This PR is a follow up to #6382

IMHO, adding the index argument should be quite natural and risk free.

I tried to keep the scope clean: Only stabilize what that PR added as unstable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.80%. Comparing base (a496da2) to head (2e7ff9f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7119      +/-   ##
==========================================
+ Coverage   94.61%   94.80%   +0.19%     
==========================================
  Files         634      618      -16     
  Lines       51801    51472     -329     
  Branches     9329     9276      -53     
==========================================
- Hits        49011    48798     -213     
+ Misses       2216     2122      -94     
+ Partials      574      552      -22     

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[fibibot]

Clean stabilization. Unstable index parameter was added in #6382 (merged Jan 2026), so it's had ~4 months of in-tree usage. All 17 unstable files removed, deno.json exports cleaned up, tests rewired to the stable imports. The implementations correctly hand off the index — sortBy reuses the loop index i, the iterator paths use index++, Array.findIndex in dropWhile's array branch already takes the index natively.

• nit: JSDoc inconsistency — sortBy, takeWhile, takeLastWhile, and firstNotNullishOf keep their pre-stabilization @param text and don't pick up the "The function receives the element and its index" wording or the "Using the index parameter" @example that the other 12 stabilized functions added. Worth aligning so users discover the new parameter from the docs.

[fibibot]

@bartlomieju this is ready to merge