feat: Mark Composite as stable

[diegohaz]

This PR marks the Composite module as stable, except the CompositeItemWidget component.

Does this PR introduce a breaking change?

This removes the unstable_ prefix from the Composite components. So this is going to break code that is currently using this module.

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[codecov]

Codecov Report

Merging #667 into next will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             next     #667   +/-   ##
=======================================
  Coverage   95.49%   95.49%           
=======================================
  Files         147      147           
  Lines        2706     2706           
  Branches      753      753           
=======================================
  Hits         2584     2584           
  Misses        122      122           

Impacted Files	Coverage Δ
...ckages/reakit/src/Composite/CompositeItemWidget.ts	100.00% <ø> (ø)
...posite/__examples__/CompositeWithTooltip/index.tsx	100.00% <ø> (ø)
.../Composite/__examples__/DynamicComposite/index.tsx	73.07% <ø> (ø)
...posite/__examples__/NestedCompositeItems/index.tsx	100.00% <ø> (ø)
...__examples__/VirtualCompositeWithTooltip/index.tsx	100.00% <ø> (ø)
...__examples__/VirtualNestedCompositeItems/index.tsx	100.00% <ø> (ø)
...kages/reakit/src/Composite/__utils/getCurrentId.ts	100.00% <ø> (ø)
...xamples__/DialogWithCompositeWithTooltip/index.tsx	100.00% <ø> (ø)
...__/DialogWithVirtualCompositeWithTooltip/index.tsx	100.00% <ø> (ø)
packages/reakit/src/Form/FormRadioGroup.tsx	94.44% <ø> (ø)
... and 16 more

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[gziolo]

I’m aware that it was unstable API and it was fine to remove it 😃

However this removal of unstable prefix l, it causes issues In WordPress packages. See WordPress/gutenberg#23332. When someone is on older version of WordPress packages that worked with Reakit 1.0, it stops working with Reakit 1.1 because references are now invalid. Would you consider keeping aliases until Reakit 2.0?