Fix indent guide styling

[A-Walrus]

Resolves #3298.
The problem was that the indent guides would render on top of whitespace, and get their theme. The incorrect looking white indent guides were actually correct according to the theme.
The first part of the fix is to make the indent guides actually use the indent guide theme.
The second part is to make indent guides a sub theme of whitespace so that when not defined they will look like whitespace, which is the desired behavior. I updated all themes that set ui.virtual.indent-guide to use ui.virtual.whitespace.indent-guide, as well as updating the documentation.

[the-mikedavis]

Instead of moving indent-guides under ui.virtual.whitespace, we could cheat a little and have a special fallback:

let indent_guide_style = theme.try_get("ui.virtual.indent-guide").unwrap_or_else(|| theme.get("ui.virtual.whitespace))

[the-mikedavis]

Looks great!

I love a fix that makes the code simpler :)

[pickfire]

Is it a good idea to patch ui.virtual.indent-guide if it exists? I think it should only patch ui.virtual.whitespace as a fallback rather than patching ui.virtual.indent-guide if it exists. I am still undecided since users may put ui.virtual.indent-guide without specifying a background.

Also, rather than doing this in a loop, I think it might be better if we patch it directly to indent_guide_style rather than doing this in the inner function.

[A-Walrus]

The reason I'm patching it with text_style is that if users only specify the foreground, it should use the background from text_style. If you look at line 457, all styles are patched from text_style

let style = spans
    .iter()
    .fold(text_style, |acc, span| acc.patch(theme.highlight(span.0)));

You're right about doing it outside of the loop, will fix.