Checkbox: add indeterminate state.

[aneeshack4]

Adding indeterminate and defaultIndeterminate props to Checkbox.

[DustyTheBot]

	Warnings
⚠️	There are no updates provided to CHANGELOG. Ensure there are no publicly visible changes introduced by this PR.

Perf comparison

Status	Scenario	Fluent TPI	Fabric TPI	Ratio	Iterations	Ticks
🔧	Avatar.Fluent	0.55	0.37	1.49:1	2000	1095
🔧	Button.Fluent	1.18	0.17	6.94:1	1000	1178
🔧	Checkbox.Fluent	1.29	0.27	4.78:1	1000	1294
🔧	Dialog.Fluent	0.31	0.16	1.94:1	5000	1541
🔧	Dropdown.Fluent	3.39	0.33	10.27:1	1000	3388
🔧	Icon.Fluent	0.24	0.04	6:1	5000	1211
🔧	Image.Fluent	0.1	0.09	1.11:1	5000	515
🔧	Slider.Fluent	1.79	0.36	4.97:1	1000	1791
🦄	Text.Fluent	0.06	0.16	0.38:1	5000	286
🦄	Tooltip.Fluent	0.33	14.67	0.02:1	5000	1673

🔧 Needs work     🎯 On target     🦄 Amazing

Generated by 🚫 dangerJS

[jurokapsiar]

Please add aria-checked="mixed" to the checbox behavior, according to https://www.w3.org/TR/wai-aria-practices-1.1/examples/checkbox/checkbox-2/checkbox-2.html

[aneeshack4]

@jurokapsiar I added it here already: https://github.com/microsoft/fluent-ui-react/pull/2241/files#diff-e844628950d9c841b914b6598457e032R16

[aneeshack4]

Note: there are several todos, this is very much a WIP. As I ramp up on this repo, I'm still figuring out several things like how to add test behavior for something like the indeterminate state to commonScreenerSteps.ts under the Checkbox. And as of now, I'm not able to see the actual results of the Screener failure beyond the error log - the tiles showing each visual regression aren't available to me so working on getting access to that too.