Add support for Javascript modules

Problem/Motivation

If you use Javascript modules with chunk-splitting via a bundler like rollup or webpack, then the imported files are not considered when generating a static file-set

Steps to reproduce

Write modern Javascript and use a bundler like rollup or webpack to package up your JS into chunks.
Include an entry point via a libraries.yml file
Run tome static
Note that the static files include the entry point, but not any of the chunks that the browser imports via the import directive

Proposed resolution

This is not dissimilar to url imports in CSS files, so we should be able to do this in a similar fashion

Remaining tasks

User interface changes

API changes

Data model changes