fix: warn when multiple React Router versions are loaded

[Turtle-Hwan]

Fixes #15356.

Problem

Loading react-router and react-router-dom versions that resolve to different
react-router module instances creates separate React contexts. Hooks can
report a generic missing-router error, while <Outlet> can render nothing
without explaining the version mismatch.

Changes

• Reuse the existing window.__reactRouterVersion marker to track versions
  seen in the browser
• Warn once in development when more than one version is detected
• Check from context-consuming hooks so both module load orders and the silent
  <Outlet> path are covered
• Preserve existing SSR and production behavior
• Add regression tests for earlier, later, and matching version registration

The patch intentionally does not attempt to bridge incompatible React contexts.
The warning tells users to align their react-router and react-router-dom
versions.

Validation

• pnpm build
• pnpm test packages/react-router/ --runInBand
  • 110 test suites passed
  • 2,214 tests passed
  • 15 tests skipped
• pnpm typecheck
• pnpm lint — 0 errors; existing warnings only
• pnpm changes:validate
• Prettier check for changed files
• Browser reproduction in Declarative and Data modes for useNavigate and
  <Outlet>