Change tabs api

[RobinMalfait]

This PR changes the API of the Tabs component. This is not considered a breaking change since we didn't release this API yet (we did publish a dev version of this).

The main reason we did this is because it looked kind of odd for the Vue implementation.

Old (React):

<Tabs>
  <Tabs.List>
    <Tabs.Tab>Tab 1</Tabs.Tab>
    <Tabs.Tab>Tab 2</Tabs.Tab>
    <Tabs.Tab>Tab 3</Tabs.Tab>
  </Tabs.List>

  <Tabs.Panels>
    <Tabs.Panel>Tab 1</Tabs.Panel>
    <Tabs.Panel>Tab 2</Tabs.Panel>
    <Tabs.Panel>Tab 3</Tabs.Panel>
  </Tabs.Panels>
</Tabs>

New (React)

<Tab.Group>
  <Tab.List>
    <Tab>Tab 1</Tab>
    <Tab>Tab 2</Tab>
    <Tab>Tab 3</Tab>
  </Tab.List>

  <Tab.Panels>
    <Tab.Panel>Tab 1</Tab.Panel>
    <Tab.Panel>Tab 2</Tab.Panel>
    <Tab.Panel>Tab 3</Tab.Panel>
  </Tab.Panels>
</Tab.Group>

---

Old (Vue):

<Tabs>
  <TabsList>
    <TabsTab>Tab 1</TabsTab>
    <TabsTab>Tab 2</TabsTab>
    <TabsTab>Tab 3</TabsTab>
  </TabsList>

  <TabsPanels>
    <TabsPanel>Tab 1</TabsPanel>
    <TabsPanel>Tab 2</TabsPanel>
    <TabsPanel>Tab 3</TabsPanel>
  </TabsPanels>
</Tabs>

New (Vue)

<TabGroup>
  <TabList>
    <Tab>Tab 1</Tab>
    <Tab>Tab 2</Tab>
    <Tab>Tab 3</Tab>
  </TabList>

  <TabPanels>
    <TabPanel>Tab 1</TabPanel>
    <TabPanel>Tab 2</TabPanel>
    <TabPanel>Tab 3</TabPanel>
  </TabPanels>
</TabGroup>

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headlessui-vue – ./packages/@headlessui-vue

🔍 Inspect: https://vercel.com/tailwindlabs/headlessui-vue/7fZmMN69faShjj3VG7gT5hnbuaWa
✅ Preview: https://headlessui-vue-git-change-tabs-api-tailwindlabs.vercel.app

headlessui-react – ./packages/@headlessui-react

🔍 Inspect: https://vercel.com/tailwindlabs/headlessui-react/3ytwHEgaAkw69WRoSaZ51BsG5rjF
✅ Preview: https://headlessui-react-git-change-tabs-api-tailwindlabs.vercel.app