Underline highlighted lines in ANSI theme

[mdibaiee]

Hey 👋
I have attempted to fix this in two ways: the first commit is a simple, ANSI-specific condition.
The second commit is using a custom scope to allow each theme to customise how the highlighted line's font style is set.

Let me know what you think and I can squash to a single final commit.

Fixes #1730

[keith-hall]

Thanks for your contribution! I like the first commit - despite it being specific for the ANSI theme and hard-coded, it is very simple. I have a few concerns about the second commit's approach, namely it seems weird to use a "custom" scope selector as a global setting, so I'd be interested to hear what the other maintainers think :)

[sharkdp]

This PR still has a change in syntaxes.bin. Should that be removed?

[mdibaiee]

@sharkdp I had overlooked that! removed that change 👍

[Enselic]

Would you mind adding a simple regression test for this change please? It is useful not only preventing regressions, but also to make it clearer what the use case is and how it works.

It would be great if you could set Approved on this when you think it looks good @keith-hall (when you have time, of course).

[sharkdp]

Approving. But I agree with @Enselic that a regression test would be great.

[mdibaiee]

Yes definitely, good call. I just added a test, and while doing so realised it actually had a bug with --style=plain (it worked with the grid style) where it wouldn't clean up the underline. 🐛 All good now.

Also moved changelog entry to Unreleased section.

[mdibaiee]

@sharkdp I think this is ready to be merged