Improve the base_path story

[pyrrho]

This PR updates how the the axum router constructed for dx serve, the --open option, and the dioxus-router WebHistory interact with the Dioxus.toml base_path field.

Prior to this change, running dx serve with a base_path set always resulted in my browser displaying the "Outside of the base path: blah" message, even when I was navigating to routes that were very definitely inside of the base path.

Three major changes are included in this PR;

• Setting a base_path now causes the dx serve router to be nested under the base path, rather than treating the base path as a route that is handled with a axum::routing::any_service.
• The --open flag now opens to localhost:8080/<base_path> rather than to localhost:8080.
• The dioxus router's default web history initializes to a URL that includes the base_path, rather than to the root.
  This last change is actually what I consider to be the most important, as it impacts dioxus web applications hosted outside of the dx serve axum server. Without this change, navigating to e.g. localhost:8080/base/path/ would cause the browser address bar to resolve to localhost:8080, even though following a Link { to: Route::Home {}, .. } would push localhost:8080/base/path/ onto the history stack.

[ealmloff]

This works much better. Thank you!

[pyrrho]

Thank you for finalizing this PR, and buttoning up the big ugly comment!