Declaration file missing plugin configs

[henrist]

#407 changed plugin.configs to be specified using Object.assign. This will not change the type of plugin, so it don't include the assigned configs. As of 0.5.2 the configs property is an empty object, preventing the use of this for type-checked ESLint config files.

I notice the ESLint documentation suggests this approach, but the ESLint project itself don't use TypeScript or emit declaration files, so I assume it's just an oversight.

I'll create a PR to fix this.