[Toolbar] POC with context checking API

[mj12albert]

Demo: https://deploy-preview-1437--base-ui.netlify.app/experiments/toolbar/text-editor

<Toolbar.Root>
  <Select.Root>
    <Select.Trigger />
    <Select.Portal>
     {/* the rest of the select */}
    </Select.Portal>
  </Select.Root>
  <Toolbar.Separator />
  <ToggleGroup>
    <Toggle value="bold" />
    <Toggle value="italics" />
    <Toggle value="underline" />
  </ToggleGroup>
  <Menu.Root>
    <Menu.Trigger />
    <Menu.Portal>
     {/* the rest of the menu */}
    </Menu.Portal>
  </Menu.Root>
</Toolbar.Root>

Full source: https://github.com/mui/base-ui/blob/2eae6834348d70804ca94c93f97c04b2f4738adf/docs/src/app/(private)/experiments/toolbar/text-editor.tsx

Forked from #1349

• I have followed (at least) the PR section of the contributing guide.

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[mj12albert]

Issues so far:

1. Seems impossible for TS to enforce the focusableWhenDisabled prop on various triggers depending on whether it's placed in a Toolbar or not
   • Input and NumberField can be toolbar items, focusableWhenDisabled will be exposed on these too?
2. Manually merging the disabled states is tricky, mergeReactProps could do it automatically but this API intentionally wants to bypass it (also requires extra code). In the demo, if the MenuTrigger is disabled (use the settings panel) aria-disabled/data-disabled are not applied correctly right now even though the trigger is functionally disabled
3. Menu/Select trigger intentionally sets tabIndex, if they are a CompositeItem, that tabIndex needs to override the one on the trigger, resulting in the same issue as 2

2/3 could possibly be worked around by not using <CompositeItem /> and using useCompositeItem directly, though it will result in more duplication

Not sure if anything can be done about 1, it doesn't seem ideal to expose focuseableWhenDisabled on regular triggers

[mj12albert]

(this is super rough right now, don't look too closely)

[mj12albert]

Going with the original API in #661