Support the table of contents

[pkgw]

Right now the table of contents "modal" in the app is a placeholder. It should be updated to actually show a table of contents! This can build on the index modal.

My idea was to generate the contents from the list of major modules, which is output by the weave program into app/src/ttw/ttw-major-module-index.js. The file app/src/named-module-index.ts has some code to manage the asynchronous loading of this information.