add allowsCustomValue to Combobox

[AlexErrant]

Closes #491

Name taken from https://react-spectrum.adobe.com/react-spectrum/ComboBox.html#custom-value

I'm seeking approval on the overall architecture of this feature before I flesh out the below 2 features. No sense in building a bunch of logic if it doesn't suit the architecture/goals of this project :)

1. It requires at least one more ComboboxBaseOptions, perhaps named customValueToOption, that takes a string (e.currentTarget.value) and returns an Option.

2. Another ComboboxBaseOptions could be a customValueValidator since it's relatively easy for users to add an emptystring Option which probably isn't desirable.

...now that I think about it some more, an even better path would be to combine the above 2 bulletpoints and call it customValueParser (shoutout to this article).

I only manually tested using this commit, perhaps more rigorous testing is required - please LMK!

[netlify]

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[jer3m01]

Hey, thanks for the PR draft!

I'll look into your suggestions and the best way to implement this.
Will get back to you soon.