Allow loading NpmPackage assets with `@StyleSheet`

[sissbruecker]

Describe your motivation

Currently it doesn't seem possible to load stylesheets from assets that have been copied over from an NpmPackage annotation using the @StyleSheet annotation.

For example, trying to load Font Awesome:

@NpmPackage(value = "@fortawesome/fontawesome-free", version = "latest, assets = {"./**:fontawesome"})  
@StyleSheet("fontawesome/css/all.css")  
public class App implements AppShellConfigurator {}

(not 100% sure what the assets path should be, the intention was to copy the whole package contents to fontawesome)

Since @StyleSheet is intended to become the standard way of including stylesheets in v25 we should aim to make this work.

Describe the solution you'd like

Including stylesheets from assets that have copied to static resources should work using @StyleSheet.

[mcollovati]

Wouldn't it make more sense to use @CssImport for npm packages assets, since they are not going to be hot reloaded anyway? Or is there a plan to remove @CssImport?

[knoobie]

Related issue: #22344

My current structure..

NpmPackage on my AppShell.. and CssImport on my MainLayout.. no additional asset configuration needed (24.8+)

@NpmPackage(value = "@fortawesome/fontawesome-free", version = "7.1.0")
public class NpmConfiguration implements AppShellConfigurator {}

@CssImport("@fortawesome/fontawesome-free/css/all.min.css")
public class MainLayout extends Div implements RouterLayout {}

[caalador]

The current path to load the asset would be @StyleSheet("/VAADIN/static/fontawesome/css/all.css")

[sissbruecker]

Wouldn't it make more sense to use @CssImport for npm packages assets, since they are not going to be hot reloaded anyway? Or is there a plan to remove @CssImport?

What would make most sense would be if we could recommend using one way of loading stylesheets, regardless where they come from, which is probably the main point of this issue. It's weird if you've only been using @StyleSheet so far, which is what we're going to recommend in the styling docs from v25 on forward, but then @StyleSheet doesn't work in this case.

Knoobie also has a point that @CssImport currently also works without having to copy any assets at all, but that is a separate topic to address IMO.