chore: Updated typings

[deskoh]

Summary

The current generated type definitions for JwtStrategy is as follows:

// jwt.d.ts
parse(req: IncomingMessage): Promise<{
  strategy: string;
  accessToken: string;
}>;

This backwards-compatible PR updates the type definition to:

 parse(req: IncomingMessage): Promise<{
    strategy: string;
    accessToken: string;
} | null>;

This is to allow other projects with strictNullChecks to sub-class JwtStrategy and able to compile without any errors.

node_modules/feathers-authentication-oidc/lib/OidcStrategy.d.ts:9:5 - error TS2416: Property 'parse' in type 'OidcStrategy' is not assignable to the same property in base type 'JWTStrategy'.
  Type '(req: IncomingMessage) => Promise<{ strategy: string; accessToken: string; } | null>' is not assignable to type '(req: IncomingMessage) => Promise<{ strategy: string; accessToken: string; }>'.
    Type 'Promise<{ strategy: string; accessToken: string; } | null>' is not assignable to type 'Promise<{ strategy: string; accessToken: string; }>'.
      Type '{ strategy: string; accessToken: string; } | null' is not assignable to type '{ strategy: string; accessToken: string; }'.
        Type 'null' is not assignable to type '{ strategy: string; accessToken: string; }'.

9     parse(req: IncomingMessage): Promise<{

[daffl]

Great, thank you! Hopefully we can eventually get everything (including the internal codebase) working with strictNullChecks, too. Will go out in the next prerelease.