feat: add isDefault method

[ruyadorno]

Add a new method isDefault(key) that enables a way to know if the
value to be returned by a config.get(key) call is coming from the
default definitions or any other different source. In case it's coming
from the default values this method returns true, it returns false
otherwise.

This addition is going to allow for effectively managing different
default values in the npm cli on a command-basis, e.g:

The save config default value in the npm cli is true, so that in
npm install and other commands that value is always going to default
to true if no user config is provided. Now let's say we want to use a
different value in npm update, for example save=false. This change
enables us to have a conditional check to see if the save config value
is coming from the default source, thus providing a way to use a
different value instead of the default:

const save = config.isDefault('save') ? false : config.get('save')

Relates to: npm/cli#4223
Relates to: npm/statusboard#324

References