Add UTF-16 ranges to JavaScript bindings

[sidmohan0]

Summary

• add one validated Core helper for converting UTF-8 byte ranges to UTF-16 code-unit ranges
• expose utf16Range on Node.js and browser WASM findings
• expose sourceUtf16Range and outputUtf16Range on transformation records and Node restoration records
• preserve the pre-Slice-8 supplied-finding input shape
• document Slice 8 as complete and update public TypeScript contracts

Verification

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features (67 passed)
• installed @datafog/node package tests
• installed @datafog/wasm package tests
• installed Python binding tests
• emoji-prefixed and JavaScript String.prototype.slice range assertions