feat(prefer-charcode-at-in-loop): add rule

[webpro]

Summary

• add opt-in prefer-charcode-at-in-loop
• report single-code-unit string comparisons inside loops when the indexed receiver is known to be a string
• provide suggestions to compare charCodeAt(index) against the numeric code unit instead of comparing one-character strings

Notes

This is suggestion-only and not part of the recommended config. It intentionally keeps unknown indexed access valid unless the receiver is a string literal/template or has string type information.

Node v24.9.0 benchmark on consumer-shaped loops:

• leading-space scan: indexed compare 759.7ms, charCodeAt compare 660.9ms
• first-character filter: indexed compare 14.7ms, charCodeAt compare 13.4ms

Test plan

• npm test -- src/rules/prefer-charcode-at-in-loop.test.ts
• npm run lint
• npm run build