The per-request context object React Router passes to loaders, actions, and
middleware as context.
React Router 8 requires this to be a real RouterContextProvider instance —
it performs a nominal instanceof check in createRequestHandler and again
in the static handler whenever route middleware is used. A structurally
equivalent object is therefore NOT sufficient, which is why the instance is
always constructed by SsrRuntime.createLoadContext, sourced from
the same react-router module the request handler came from.
get<T>(key: RouterContextKey<T>): T
Reads a key, falling back to the key's defaultValue.
set<T>(key: RouterContextKey<T>,value: T): void
Writes a key, overwriting any previous value.