Function deepMerge

deepMerge(a: object, b: object): object
Creates a new object by merging object b onto object a, biased toward b:
- Fields in b overwrite fields in a.
- Fields in b that don't exist in a are created.
- Fields in a that don't exist in b are left unchanged.
Parameters
- a: object
  The base object.
- b: object
  The overriding object.
Returns object
A new merged object.

Throws
If either a or b is not a plain object.