Fix issue with `patternProperties` and `strictIndexSignatures` option

[yorinasub17]

Fixes https://github.com/bcherny/json-schema-to-typescript/issues/559

See issue for details.

NOTE

This is my first time contributing here! I must admit that I do not fully grok the intricacies around the type issues as described in https://github.com/bcherny/json-schema-to-typescript/issues/235 , so I am not 100% certain this is the right fix. That said, this does address my compilation issues when generating from schemas in https://www.schemastore.org/json/.

Also, I am not familiar with ava and snapshot testing, so I am not 100% sure I rebuilt the snapshot correctly, so pointers on how to regenerate it would be helpful if I did indeed do something wrong!