fix: Shift route-announcer to top of layout parent (#44853)
Without `top: 0` the route announcer triggers subtle layout shift in
Blink/Chrome.
BTW, this mostly (possibly only?) occurs on pages with specific overflow
proprty combinations on `<html/>` and `<body/>`, and only when rendering
inside a custom element like the `<next-route-announcer />` portal.
(Changing the portal into `<div/>` seems to make the layout shift
disappear, but that would be far more invasive change than just adding
`top:0`.)
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Co-authored-by: Shu Ding <g@shud.in>
