restore background highlight in diff of assertEquals

[kt3k]

We temporarily disabled background highlight in diff of assertEquals (#2601) as it's very hard to see in certain terminals

We used to use bgGreen and white combination for that highlight. It's enough visible in some terminals, but it's not in some others. (See #2575 for example)

[kt3k]

One solution might be to use true color coloring when the terminal supports it, and fallback to ansi color when no support of it

Note: supports-color npm module seems doing this detection

[luk3skyw4lker]

So I could only check if the terminal supports true color and if it does I can color it with bgGreen and white and if it doesn't leave it uncolored?

[kt3k]

The issue happens on some terminals with ansi colors (green, bgGreen, etc). To avoid that if the terminal supports true color then we should use color code (like 006400 for green, for example), and if it doesn't, we fallback to ansi colors (green, bgGreen, etc)

[luk3skyw4lker]

I'll take a stab at this one then. I'll be trying to open a PR next weekend

[lino-levan]

This becomes trivial when denoland/deno#18549 lands.