build: package theme templates and static assets

[clarete]

There's currently no way to separate a list of files to be exported from "source" files. That affects mostly HTML templates and built CSS files.

The files we'd want to to package as final theme "artifacts" need to be built from the source files and made them available for users to:

1. be able to import templates that are located within the weblorg package;
2. be able to copy static assets from themes within the package to the user's static site instance directory

I'd suggest creating something like a subdirectory src within each theme so we can easily exclude it with globs and such. Or we can just identify which files are to be exported and put them within a sort of an allow-list? That's probably relevant to the melpa package.