[api-extractor] More predictable merging of arrays in configuration

[Josmithr]

Fix for #4786

Merging of API-Extractor configurations leverages Lodash's merge functionality. For object / primitive properties, the resulting behavior is sensible. But for array properties, Lodash's merge behavior is very unintuitive.

Consider a base API-Extractor configuration with the following properties:

{
  "apiReport": {
    "enabled": true,
    "reportVariants": ["alpha", "beta", "public"]
  },
}

And a derived configuration with the following properties:

{
  "apiReport": {
    "reportVariants": ["complete"]
  },
}

What would you expect the merged configuration to look like? There are a couple of reasonable options here, but Lodash would yield:

{
  "apiReport": {
    "enabled": true,
    "reportVariants": ["complete", "beta", "public"]
  },
}

Where each value is overridden on a by-index basis.

This is incredibly unintuitive behavior. Per the discussion in #4786, the merge logic has been updated such that array properties in derived configurations override array properties in base configurations. So for our example above, the merged configuration would look like:

{
  "apiReport": {
    "enabled": true,
    "reportVariants": ["complete"]
  },
}

Which is a predictable and intelligible behavior.

In the future, we may want to offer more user options for how to merge arrays, but in the short term this solution prevents confusing and error-prone behavior.