Typescript ESType definition improvements

Hi,

The following types can be improved on to be more correct in the lib.es5.d.ts file.

These type improve typically deal with the cases on of the arrays items keys has subset of the other keys, in this case the results is the common subset, with mismatching fields made optional.

This requires functionality in typelevel-ts, however, that is only based on keys.
new methods should do and K in keyof T, T[K] extends S[K] ? K : never

// Improved more correctly helper method.
type ObjectOmit<T extends Record<string,any>, O extends Record<string,any>, KeyOfO keyof O> = Pick<T, {
 [K in keyof T] : K extends KeyOfO ? 
                         T[K] extends O[K] | undefined ? never : K // may need a two way extends comparison here.
                          : K
}[keyof T]

export function ObjectAssign<T extends Record<string,any>, U extends Record<string,any>>(target: T, source: U): 
ObjectOverwrite<T, U>
{
  return Object.assign(target, source);
}

export function ArrayConcat<A>(arrayA: A [] , arrayB : A []) : A[]

export function ArrayConcat<B, A extends B>(arrayA: A [], arrayB : B []) : B & Partial<ObjectOmit<A, keyof B>>[]
export function ArrayConcat<A, B extends A>(arrayA: A [], arrayB : B []) : A & Partial<ObjectOmit<B, keyof A>>[]
{
  return arrayA.concat(arrayB) as any
}


export function FilterDefined<T>(items : Array<T> | undefined) : NonNullable<T> []
{
  if (items)
    return items.filter(f => f != undefined) as NonNullable<T> [];
  else
    return []; 
}

Comienza leyendo lib.es5.d.ts y la funcionalidad basada en claves de typelevel-ts mencionada en el issue. Revisa las firmas propuestas de ObjectAssign, ArrayConcat y FilterDefined y verifica después que las declaraciones gestionen las claves comunes, las discrepancias opcionales y el filtrado de undefined según lo previsto.