feat(web): ModelCompositor enhancements

From #1851:

Enhancements for future PRs

> * using a generalized "tag" mechanism for suggestions and nailing down _exactly_ what those semantics are

While we currently support a single tag ('keep'), others are planned for the future.

> * Spacing in the model compositor

A baseline for Roman script languages is in place, but doing regex checks against the right Unicode class was breaking IE and preventing the more robust implementation.