`mainFields` type mismatch with webpack

[Aaron Adams]

This plugin:

tsconfig-paths-webpack-plugin/src/options.ts

Line 12 in cb0f1dd

 readonly mainFields: string[]; 

Webpack:

https://github.com/webpack/webpack/blob/2738eebc7880835d88c727d364ad37f3ec557593/types.d.ts#L9718

mainFields?: (string | string[])[];

This causes a type mismatch when forwarding config.resolve.mainFields to this plugin:

It might be preferable to directly pass through the types of mainFields (and extensions) directly from webpack. Alternatively, typing them as unknown would prevent type checking altogether, which seems acceptable in this pass-through scenario.