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 []; 
}

Start by reading lib.es5.d.ts and the typelevel-ts key-based functionality mentioned in the issue. Review the proposed ObjectAssign, ArrayConcat, and FilterDefined signatures, then verify that the declarations handle common keys, optional mismatches, and undefined filtering as intended.