Expose zero-width spaces

…any maybe other non-printing characters.

Zero-width spaces are typically "invisible" (non-printing), but can be used for [fingerprinting](https://medium.com/@umpox/be-careful-what-you-copy-invisibly-inserting-usernames-into-text-with-zero-width-characters-18b4e6f17b66) or just [making data invalid](https://github.com/elementary/icons/pull/834). Ideally Code would highlight otherwise non-printing characters so developers were aware of the contents of their code files.

For reference, some tools like git diff or vim show them.