Feature: Highlight environment variables that are not set

fish, version 3.4.1
Linux userA-dev 5.4.0-113-generic #127-Ubuntu SMP Wed May 18 14:30:56 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
xterm-256color

## Feature request

Consider this example:
```fish
helm pull https://some.com/path/helmchart.tgz --username $FOO --password $MOO
```

I have accidentally pulled up this line from my history, but forgot I had removed the `$MOO` variable. `$FOO` and `$MOO` is of another color:


To me it would be quite cool if fish could show these as crossed-over text, or of another much more faded color, or similar. To show me that those variables do not exist and are unset. That would have helped me find my fault. When I executed the above command I did get a 401 back but I never figured it was because they were unset.