Sass support with dotnet watch

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

My current specific problem is that I want to use SASS (specifically, `Component.razor.scss` files) in my Blazor WebAssembly projects. I have already managed to make this work for regular builds and `dotnet watch` full rebuilds by adding this to my project:

```

compressed
expanded

```

This builds the CSS files in the right places and everything just seems to work -- for full builds. (Although with the caveat that you need to put them into `.gitignore`, since it doesn't appear to be possible to build them to an intermediate directory instead.)

But with a `dotnet watch` running, editing and saving the `.scss` file is just seen as a "static file" and it reloads without actually building the CSS files, so I have to force a full rebuild.

### Describe the solution you'd like

This is mostly a repeat of #39609 (which was closed and locked from lack of discussion, so I am posting a new issue as the bot requested).

It would be good to get some documentation on how to integrate additional tools with the hot reload process, either via tools that have their own built-in watch functionality or (probably simpler) via some project rules that can express "if these file patterns change then run this build command/target before reloading"; or just a documented target that can be hooked to run additional build targets, perhaps conditionally on the changed file list. The expectation is that these would mostly be for static assets, but perhaps source generators could be supported too?

Or alternatively, to just include sass support as a first-class citizen (which is #24420). But then this question will come up again next time someone wants to use some other tool.

### Additional context

_No response_