feat(react): add indeterminate support to checkbox

[chornonoh-vova]

closes: #479

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[scurker]

This is a great start!

As an overview for testing, please take a look at the testing strategies guidelines as there's a few things we're missing.

Additionally, it's not documented (yet) but we're capturing screenshots for all interactive elements. We should include the indeterminate checkbox as an additional state to capture here: https://github.com/dequelabs/cauldron/blob/develop/packages/react/src/components/Checkbox/screenshots.e2e.tsx

[scurker]

One additional thing that I'm realizing is we're not matching what native checkboxes do.

A checkbox can have an indeterminate state and be checked / unchecked. When clicked, the indeterminate state is removed and the checked value is flipped from its current value so we will likely want to match that behavior as our current checkbox component isn't "controlled".

[scurker]

At a high level, this looks good - thanks for making changes. I will give it a final pass over tomorrow morning.

[scurker]

Very very minor nit, otherwise looks good!

[shawnsharpDQ]

Is there anything we can do for the visibility of the disabled indeterminate state?

compared to the disabled checked