Add a way to set project root to something else than current working directory

[mb21]

This might be needed if you're in a monorepo or for some other reason cannot cd to the root project dir before running Mastro.

tsc has the --rootDir and Astro the --root CLI option. We could do something like that for the serve and generate commands. Alternatively, we could also use an env variable for this, but that seems less explicit?

List of functions in Mastro that would need to read this out:

• findFiles (used e.g. by the file-based router in loadFileBasedRoutes)
• ?