feat: indeterminate Checkbox

[jaulz]

Motivation

Implement indeterminate Checkboxes according to:
https://material.io/design/components/selection-controls.html#checkboxes

Test plan

[callstack-bot]

Hey @jaulz, thank you for your pull request 🤗. The documentation from this branch can be viewed here.

[satya164]

Hey! Thanks for the PR. I think instead of having 2 props, having a single prop would be better, because checked and indeterminate don't mean anything together because indeterminate means that the state is unknown.

status: 'checked' | 'unchecked' | 'indeterminate'

We should do the same for radio :)

[jaulz]

Hm, I'm not sure if that's a good idea from a developer perspective since he needs to handle strings instead of truthy/falsey values.

[satya164]

It's a minor inconvenience, but better than an inconsistent API IMO. Having two separate props which control the same thing also means that people who actually use the indeterminate will have to deal with 2 props in all places which can be cumbersome, and if someone wants to add this behavior to existing app, it'll require more refactoring.

It's also easier to introduce bugs when 2 props control the same state.

[jaulz]

@satya164 just implemented the changes as suggested :) The indeterminate status for radio buttons is by the way not part of the specs (https://material.io/design/components/selection-controls.html#radio-buttons).

[ferrannp]

@satya164 can you re-check?

[jaulz]

@satya164 any chance to have a look at it for the 2.0 release?

[satya164]

merged in 3b09171