Create a function for deep assign of string typed keys.

[fidel-ml]

The function Object.assign works on a shallow level, assigning only strings on the first level.
When nested objects are used, a similar functionality but working on any depth level may be handy.

For example:

   interface A {
       a1: string;
       a2: {
           a21: string;
           a22: string;
       };
   }
   const defaultA: A = { a1: 'Default A1', a2: { a21: 'Default A21', a22: 'Default A22' } };
   const ej1: A = deepStringAssign<A>(defaultA, { a2: { a21: 'User A21' } });
   // EXPECTED: ej1 === { a1: 'Default A1', a2: { a21: 'User A21', a22: 'Default A22' } };
   const ej2: A = deepStringAssign<A>(defaultA, { a1: 'User A1' },
                                                { a2: { a22: 'User A22' } });
   // EXPECTED: ej2 === { a1: 'User A1', a2: { a21: 'Default A21', a22: 'User A22' } };