feat: Switch routable props type over to an xor

[rschristian]

This change ensures the user sets either path or default for a route, but never both.

Previously, the types erroneously allowed you to set neither &/or both; setting neither made it a dead route, never to be used, and setting both is simply redundant.

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TS's error message isn't great, if the user sets neither, they'll be met with the following which only mentions the latter half of the XOR:

Type '{ component: () => Element; }' is not assignable to type 'IntrinsicAttributes & (RouteProps<{ component: () => Element; }> & Partial<{ component: () => Element; }>)'.
  Type '{ component: () => Element; }' is not assignable to type 'IntrinsicAttributes & { path?: never; default: boolean; } & { component: AnyComponent<{ component: () => Element; }>; } & Partial<{ component: () => Element; }>'.
    Property 'default' is missing in type '{ component: () => Element; }' but required in type '{ path?: never; default: boolean; }'.

Related information:

  * router.d.ts#34,20: 'default' is declared here.

 (tsserver 2322)

Don't think we can do anything about this though.