Added the indexer to the KnockoutValidationRuleDefinitions interface

There is currently no indexer present on the `KnockoutValidationRuleDefinitions` interface.

This results in `error TS7017: Index signature of object type implicitly has an 'any' type` when custom rules are added according to the [Custom Validation Rules section of the Knockout-Validation documentation](https://github.com/Knockout-Contrib/Knockout-Validation/wiki/Custom-Validation-Rules) and the file is compiled with the `--noImplicitAny` flag.

Adding the indexer to the interface allows the rules to be added according to the documentation.