Dims text by a certain factor - this respects theme opacity. The amount
is documented in the code. As far as I can tell, all other terminals
also dim text using this same method. Dim only affects the foreground.

<img width="755" alt="SCR-20240209-mfls"
src="https://github.com/zed-industries/zed/assets/52195359/c32f2aff-1142-4333-a05d-6aca425cb235">

Release Notes:

- Added terminal text dimming (fixes #7497)