Add --fallback-url option to dev server

[caseyWebb]

Hey! 👋🏼

This PR adds a --fallback-url=<http://127.0.0.1:3000> option to elm-pages dev.

For my use-case, this allows proxying to an existing legacy app for unresolved requests (and handling 404s there), rather than hitting the legacy app first and proxying to the dev server.

For some context, I'm using an adapter like this for the build/prod version

express()
  .use(express.static(elmPagesDistDir))
  .use('/app', elmPagesSSR)
  .use(createProxyMiddleware({ target: 'http://127.0.0.1:3000' }))

where all of my elm-pages routes are behind /app (and there are not any /app routes in the fallback app, OK to use elm-pages 404 page–may make sense to expand this to cover that use case though and forgo elm-pages 404 handling entirely?)

Happy to make any changes!

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