Allow for partial overrides of Svelte files/components (with a build process)

Problem/Motivation

Ben has come up with a way to read override files, based on naming convention, to override just the markup, CSS, or JS (or the entire file) of a .svelte file, and during the build step output an app that includes those overrides. However, per discussion at DrupalCon NA 2023, we haven't yet identified an actual need for this. So, this is postponed pending user feedback. Per Lauri, don't provide a feature "just in case."

Proposed resolution

See discussion in parent issue.