Work with Keystone folks to enable TypeScript strict mode on `@keystonehq/ur-registry`

In adding support for TypeScript to the codebase, we plan on enabling [strict mode](https://www.typescriptlang.org/tsconfig#strict), which is a setting in `tsconfig.json` that the TypeScript team recommends which enables a bunch of other settings designed to promote higher code quality/design and catch more runtime errors.

The problem here is that if _we_ enable strict mode, all of the packages we use that are written in TypeScript also have to have been developed with strict mode as well. This seems to be the case for the packages we use so far but for one: Keystone's `ur-registry` package. There is an [issue open in the repo for this package](https://github.com/KeystoneHQ/ur-registry/issues/13) to track enabling of strict mode, but this may involve fixing some issues in the codebase itself. We need to work with the Keystone team to do this in a way that doesn't break backward compatibility.

Start with the TypeScript strict setting in tsconfig.json and review KeystoneHQ/ur-registry issue 13 for the package's compatibility work. Coordinate with the Keystone team on the required fixes, then verify strict mode can be enabled without breaking backward compatibility.

Written by the indexing model from the issue text.