Support dimmed out code in the ty playground

[sharkdp]

We now support highlighting code in a "dimmed out" font for unreachable blocks and unusued variables. It would be great if this were supported in the ty playground as well.

[MichaReiser]

Posting here in case someone finds it useful:

I had codex do some of the changes, but I still have to clean up the PR (). Happy to merge someone else's PR if they beat me to it :)