Add better handling for route collisions

[BryceRussell]

Currently, when creating a theme using ATP, authors may end up creating routes that collide with routes from a user or integration. ATP should add better handling for routes that collide with each other.

Things to consider:

• Theme routes have a higher priority than user defined routes, should user defined routes have a higher priority instead? ATP could prevent the injection of a route if it already exists inside the user's project.
• Should ATP log or warn about colliding routes and explain how to fix?

Note: ATP injects routes inside the astro:config:setup hook which runs before Astro can provide any information about routes inside the project. The only way to get information about a user's routes it to read the file structure of the src/pages directory.