wip: dynamic handler types to enforce return value in Hono handlers

hono_handler.ts

// Handler type when `next` is NOT provided and a response is expected
export type HandlerWithoutNext<E extends Env = any, P extends string = any, I extends Input = BlankInput, R extends HandlerResponse<any> = any> =
    (c: Context<E, P, I>) => R;

// Handler type when `next` is provided
export type HandlerWithNext<E extends Env = any, P extends string = any, I extends Input = BlankInput, R extends HandlerResponse<any> = any> =
    (c: Context<E, P, I>, next: Next) => void;

// Union type to combine both handler types
export type Handler<E extends Env = any, P extends string = any, I extends Input = BlankInput, R extends HandlerResponse<any> = any> = HandlerWithoutNext<E, P, I, R> | HandlerWithNext<E, P, I, R>;