`cfg` highlighting is inconsistent

[schungx]

The newest version has cfg highlighting, essentially graying out the code shielded by an inactive feature gate.

This is a great feature, except that I find it inconsistent.

First I'd load a file, and it would color fine. Then I'll switch to another file, and it won't color at all. Then I switch back to the original file, and it'll lose the coloring.

Sometimes it'll suddenly come back after a while, sometimes it won't.

Usually when I restart Visual Studio Code, it'll work fine the first time. Then not afterwards.

[schungx]

I did some more experiments and it seems to work for any document window already open when I start VS Code. Any window I open afterwards, or closing the original one and reopening, will cause it not to color.

[jonas-schievink]

This is a general problem with diagnostics. I'm not sure why it happens though, it might be a VS Code bug.

[jonas-schievink]

Found the issue, and opened a PR that should fix it in #6637