leptos-shade frontend assets build

`leptos-shade` is open source at https://gitlab.com/dostuff/leptos-shade

Currently we have to have users inject some code into their build.rs to export the SCSS modules to the location their asset pipeline expects.

I don't mind if configuration is required for now but having users:

1. Add a `build.rs` to a project that might not otherwise have one
2. Add dependency-specific code to that `build.rs`

doesn't really satisfy me. I'm looking for alternatives. This is a problem the Node/NPM ecosystem have solved over and over, so I'm wondering which of those frontend asset build tooling options look most translatable to what we have in an otherwise Rust-only build system.

Start with the build.rs injection described in the issue and trace how SCSS modules are exported for the asset pipeline. Compare the Node/NPM tooling options under consideration with the Rust-only build constraints; done means selecting and documenting a concrete alternative that removes dependency-specific build.rs code.

Written by the indexing model from the issue text.