feat(essentials): better yarn config commands

[paul-soporan]

This PR has been updated to include the suggestions @arcanis gave me on Discord.

What's the problem this PR addresses?

Currently:

• yarn config get doesn't correctly display values that aren't strings
• yarn config set can only set string values

How did you fix it?

I made yarn config get display configuration values using inspect or as JSON when using the --json flag.
For displaying them as JSON, I had to implement a function that recursively converts maps to objects.

---

I made yarn config set accept values as JSON when using the --json flag.
When not using the --json flag, values are set as strings.
When using the --json flag, values are parsed as JSON before being set.

TODO:

• update tests for the yarn config get command
• add examples for the new features
• maybe colorize the config option name in the yarn config set output. I'll let @arcanis handle this if he wants, as I'm not good at picking colors 😄

[paul-soporan]

@arcanis, I'm wondering if we should make the yarn config get <option> --json output also include the name of the option, to make it more easily parseable.

[arcanis]

Awesome! Just a little typeof value === 'string' to add to yarn config get and it'll be good to go! 😃

[paul-soporan]

BTW, shouldn't we remove the polyfills for Node 8, since we dropped support for Node 8?

[arcanis]

Yep, finally 🤡