[dialogs] Add iOS scroll lock elements

[atomiks]

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

Closes #1893

This solution seems to be robust in that it doesn't break input focus slide/have glitch issues with animations.

Drawbacks

1. Three-four extra wrapper elements are required just to get the behavior to work. I added disableScrollLockElements as a way to customize them
2. Trying to style <Dialog.Viewport> as a flex container for <Dialog.Popup> doesn't work because of the nesting, breaking expectations

Note that for Popover it doesn't solve the use case listed here #3100, they'd have to specify these elements manually. We could document this? <Popover.Viewport> is also conceptually different from <Dialog.Viewport> (cc: @colmtuite)

Alternatives

1. Document "Locking scroll on iOS" as a separate section
2. Create a separate component on iOS called <ScrollLock> solely for the docs (copy-pasteable) so the users can edit the elements and see that <Dialog.Viewport> cannot be a flex container for <Dialog.Popup>. Reveals how to solve the Popover use case in #3100 as well.
3. Re-adopt usePreventScroll from RA which seems to have fixed most related issues

[pkg-pr-new[bot]]

vite-css-base-ui-example

pnpm add https://pkg.pr.new/mui/base-ui/@base-ui-components/react@3183

pnpm add https://pkg.pr.new/mui/base-ui/@base-ui-components/utils@3183

commit: 664ef97

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui-components/react	🔺+401B(+0.10%)	🔺+137B(+0.11%)

Details of bundle changes

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