feat(react-server): router transition state

[hi-ogawa]

todo

• expose isPending
  • separate ones for RSC and action?
  • action state is already managed by react https://react.dev/reference/react-dom/hooks/useFormStatus but this is available only locally, so we probably want something global one. test(react-server): test ReactDom.useFormStatus #225
  • back/forward navigation doesn't trigger transition?
    • for now, setTimeout to delay rsc transition after useRouter re-rendering?
    • can we implement "pending status" on user land without useTransition?
  • example global progress
    • animation works differently with StrictMode (head/body hoisting is working differently?)
      • well, it looks like it's fine now? I don't know what was happening anymore...
• hooks for browser navigation state for client component (e.g. current url)
• refactor context (later)
  • tanstack-style refined state subscription? refactor(react-server): simplify router store #227
• test
• simplify browser root component (setRsc, updateRscByFetch, callServer, etc...?) still hacky but fine...
• cloudflare workers crash?
  • handle undefined process.env.DEBUG