Add TypeScript types for validation

**Feature request**
It would be great to have backup for parsers.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_backup` with default `false`.

The issue names no files, tests, or entry points. Start by locating the configuration parsing and validation code, then determine how TypeScript types relate to the proposed `enable_backup` option and parser backup behavior. Done means the requested types and configuration behavior are defined consistently, with validation covering the default `false` value.

Written by the indexing model from the issue text.