Improve light colour highlight

[alanmcruickshank]

Ok, so this might be a little controversial.

I understand why #5458 was merged - the previous situation didn't work well with dark mode terminals (see #1659 and #1170). However I'm finding that the solution of changing the background is also quite hard to read on some other themes (in particular on powershell), so I've played with a few options, and I think this might be an improvement on both.

This changes the lightgrey colour to just light and maps it to the ANSI yellow code. It does change the overall feel quite a bit, but I think it's a good compromise. Here's some shots of what it looks like on a few different colour schemes:

Ubuntu

Monokai

Solarized

Unfortunately I don't have a mac and so can't confirm exactly what it looks like on MacOS dark solarized as per the original request.

@ryaminal , @louis-vines, @puetsch - as the original posters of the linked issues, are you able to try this variant and confirm whether it's readable on MacOS dark mode? @WittierDinosaur if you're on a Mac, you might also be able to confirm?

[github-actions]

Coverage Results ✅

Name    Stmts   Miss  Cover   Missing
-------------------------------------
TOTAL   17551      0   100%

225 files skipped due to complete coverage.

[coveralls]

coverage: 99.985%. remained the same
when pulling f266ada on ac/colours
into fbf8fad on main.

[ryaminal]

@alanmcruickshank, looks good to me. thanks for actually fixing the problem instead of bandaging it like i did. :)

[WittierDinosaur]

MacOS

[WittierDinosaur]

LGTM