Add no-for-loop-extra-variable rule for cached array-length loops

[juarezsousa-ctrl]

Fixes #295

This adds a new unicorn/no-for-loop-extra-variable rule to handle the specific cached array-length for loop shape discussed in #295.

Context:

• A case not handled by no-for-loop #295 identified that no-for-loop does not report on loops like for (let i = 0, j = array.length; i < j; i += 1).
• Add no-for-loop-extra-variable rule #492 explored a separate rule for this, but it grew into a broader implementation and was never finished.

This PR takes a narrower approach:

• only targets the conservative i = 0, j = array.length pattern
• only fixes local, unambiguous cases
• skips ambiguous cases instead of trying to hoist or preserve cross-scope usages
• rewrites the loop into a simpler form so existing rules can operate on it afterward

no-for-loop itself is left unchanged in this PR.

Included:

• new no-for-loop-extra-variable rule
• dedicated tests for supported and skipped cases
• rule export wiring
• docs/readme entry

[juarezsousa-ctrl]

Rebased on latest main, resolved the conflict, and reran the checks.

All checks are passing now.