Improve type checking for notification and request types

[dhuebner]

When using request and notification types the types are not properly checked. Example:

const Notification: NotificationType<string> = {
    method: 'note'
};

messenger
    .sendNotification<number>(Notification, { type: 'webview', webviewType: 'test' }, 5);

const Request: RequestType<string,string> = {
    method: 'req'
};

//  no error reported below, but should as Request parameter type was redefined from <string,string> to <number, number>.
const response = messenger.sendRequest<number, number>(Request, { type: 'webview', webviewType: 'test' }, 5);

[dhuebner]

Fixed with #15