layout: Implement node geometry queries against `BoxTree`'s `Fragment`

This is a followup to #36629, continuing to implement script-based
layout queries  using the `Fragment`s attached to the `BoxTree`. In this
change, geometry queris (apart from parent offset) are calculated using
`Fragment`s hanging of the `BoxTree`.

In order to make this work, all `Fragment`s for inlines split by blocks,
need to be accessible in the `BoxTree`. This required some changes to
the way that box tree items were stored in DOM `BoxSlot`s. Now every
inline level item can have more than a single `BoxTree` item. These are
carefully collected by the `InlineFormattingContextBuilder` -- currently
a bit fragile, but with more documentation.

Co-authored-by: Oriol Brufau <obrufau@igalia.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>

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