[autocomplete][combobox] Support external items mapping

[atomiks]

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

This is an attempt to support the cmdk library's composition style, without items specified on Root, and without needing to perform filtering externally:

<Combobox.Root>
  <Combobox.Input />
  <Combobox.Portal>
    <Combobox.Positioner>
      <Combobox.Popup>
        <Combobox.List>
          {fruits.map((item) => (
            <Combobox.Item key={item} value={item}>
              <Combobox.ItemIndicator>
                <CheckIcon />
              </Combobox.ItemIndicator>
              <div>{item}</div>
            </Combobox.Item>
          ))}
        </Combobox.List>
      </Combobox.Popup>
    </Combobox.Positioner>
  </Combobox.Portal>
</Combobox.Root>

Right now, Autocomplete supports this well, but you need to filter items externally yourself (via Autocomplete.useFilter). Combobox has limited/partial support and does not work fully.

Implementation

Each item returns null internally when it's not rendered. Groups use hidden attribute to prevent unmounting the children so their effects still run.

Pros

• You can use this in a server component without re-exporting a client component as there's no function-as-child pattern
• There's no need to explicitly type the item type in the function child (since TS infers it from the external array, naturally)
• More flexible item rendering/ordering

Cons

• Extra code to handle
• As with Select, Combobox needs to force-mount the popup when interacted with because it needs to register the items/labels since the rest of the component tree doesn't have access to items
• Users must use objects for each item for the label to be known ahead of time (before force-mounting)
• Virtualization requires items. Switching from no virtualization to virtualization already works naturally with the current/default API

[pkg-pr-new]

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@3594
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@3594
  

commit: 9204b7b

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+2.13KB(+0.52%)	🔺+774B(+0.59%)

Details of bundle changes

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[KingSora]

@atomiks do you think this would also cover the usecase described here: #3559 ?