feat(es2026): Error.isError static method documentation (#1872)

Closes #1872 Added `[ES2026] Error.isError` section to the end of the error handling chapter, following the research notes in `docs/research/es2026/error-is-error.md`. Changes: - Added a short section after `[ES2022] Error Cause` introducing `Error.isError` - Updated the summary section to mention `Error.isError` - Existing `instanceof` examples are unchanged (type checking still uses `instanceof`)