gpui: Add container_query element

[mikayla-maki]

Adds a container_query element to GPUI, in the spirit of CSS container queries: the element's own size is determined solely by its style and the space offered by its parent, and once that size is known the provided closure is called with the measured size to build the contents.

container_query(|size, _window, _cx| {
    if size.width < px(240.) {
        div().child("Narrow layout")
    } else {
        div().child("Wide layout")
    }
});

Implementation notes:

• Defaults to filling its parent (size_full() semantics), overridable via Styled since contents can't influence the container's size (the same constraint CSS container queries impose).
• Reworks the grid_layout example (the Holy Grail layout) to demonstrate it: the three-column grid collapses to a stacked column when the window is narrower than 400px, and the header shows the live measured width.

Release Notes:

• N/A