A React Router context key, used by identity as the argument to
RouterLoadContext.get / RouterLoadContext.set.
Structurally identical to React Router's RouterContext<T>, so a key created
by this package and a key created by createContext<T>() from react-router
are interchangeable. defaultValue is returned by get() when no value has
been set; a key without one makes get() throw for an unset key.
readonly
optional
defaultValue: T
Value returned by get() when the key has not been set.