Remove the page/layout distinction from RouteTree (#98970)
Part of a series of internal refactors to improve param tracking on the
client.
A route tree node used to come in two variants, a layout and a page,
distinguished by an isPage flag that every constructor had to keep
consistent with the VaryPath it built. The variants only differed in
whether the vary path carried a search params entry, and the code that
needed to know either checked the flag or read the vary path's shape
positionally.
Collapse them into one RouteTree type with one VaryPath type. The few
places that still care whether a node is a page derive it from the
segment. When something needs to read the search params, they can be
accessed from the RouteTree or VaryPath like we do for other params.
This is a pure refactor of the existing code. It reads more naturally
with a single tree type and a single vary path type, which the rest of
this stack builds on.
