Addon-knobs: Fix select options types to allow string[] and null

[emilio-martinez]

Issue: #7348

What I did

Fixed select(...) knob types to allow for:

• Values of type string, string[] and null.
• Options passed as string[] in addition to objects.

This better aligns with the Knobs Addons select docs, i.e.:

const options = {
  Red: 'red',
  Blue: 'blue',
  Yellow: 'yellow',
  Rainbow: ['red', 'orange', 'etc'],
  None: null,
};

...

You can also provide options as an array like this: ['red', 'blue', 'yellow']

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[kroeder]

Are select() knobs always strings? Is select(1, 2, 3, 4) not possible?

Also, if you allow nulll.. is the intention to pass in a null value as knob, like that select(null, 'string')?
If yes, this could be useful too: select(null, undefined, 'string')

[emilio-martinez]

Good points! You're right, technically numbers should be fine because they'd pass the strict eq. comparison. I had added null because it was in the example, and similarly undefined should work.

Updated. Let me know your thoughts!

[shilman]

LGTM