Add new sub-module to Tome Sync that automatically deletes unused files

Problem/Motivation

While the "drush tome:clean-files" command and recently added form are great, it's easy to forget to clear files before committing to Git. It'd be nice if files were deleted immediately after they stopped being used, which could cause data loss and would break reverting revisions (unless you re-saved the file after a revert), but that might be OK for a Tome user that commits often and only works locally.

Steps to reproduce

n/a

Proposed resolution

Add a new sub-module that does this.

Remaining tasks

Review/commit patch.

User interface changes

None.

API changes

None.

Data model changes

None.