fix: improve flag types

[mdonnalley]

Because of how globalFlags is typed, if you do not define any global flags then the flags returned from Command.parse will include { [flag: string]: unknown }, which prevents typescript from erroring when you're attempting to access a non-existent flag.

This PR changes the default type for flags and globalFlags from { [flag: string]: unknown } to object. Doing so removes the index signature and allows typescript to fail when accessing a non-existent flag. Unfortunately, this requires using the object type which is discouraged.

The alternative to this wrapping the returned flags in a new generic type that removes the index signature,

type Knowable<T> = {
   [ K in keyof T as string extends K ? never : number extends K ? never : K ] : T[K]
};

This also solves the problem but it slightly obscures what the actual flags are, e.g.

[mshanemc]

yay, object