Make the FS router support lazy loading

[Legioth]

Describe your motivation

In a big application, views should be split up into separate JS bundles so that the main bundle doesn't keep growing as more views are added.

Describe the solution you'd like

Add a lazy: boolean field to ViewConfig. The default value is false for a view mapped to / or /login and true for all other views. (Yes, this is yet another property where the default is true but that's fine-ish since lazy is much more descriptive than any other name that I've considered).

For a view that is lazy (either explicitly configured or based on the default), we generate file-routes.ts to do const ViewName = lazy(() => import("../views/view-name.js")) instead of the current import * as ViewName from "../views/view-name.js".

Describe alternatives you've considered

We might also want to make it possible to group multiple related views into a single import instead of creating a separate import for each view. We can add that later by changing the type of lazy to boolean | string and changing the logic for generating file-routes.ts so that all views using the same lazy string value are imported from the same URL (by generating a separate helper file that imports and re-exports all desired views).

Additional context

No response