feat(prefer-slice-over-split-index): add rule

[webpro]

Summary

Add prefer-slice-over-split-index to report split(<string>)[0] and split(<string>)[1] when only one piece is needed.

The rule targets non-empty string separators and skips regex separators, unknown separators, empty-string splits, and split limits that change the indexed result.

This is opt-in only and is not enabled in the recommended/performance config by default.

A quick benchmark on Node 24 showed equivalent indexOf()/slice() forms were roughly 3x-4.5x faster than split()[0|1].

Verification

• npm test -- src/rules/prefer-slice-over-split-index.test.ts
• npm run lint
• npm run build
• npm test