Support setting custom background color for syntax highlighting (#12400)

Release Notes:

- Added support for `background_color` in `syntax` map in `theme.json`.

This adds support for setting a `background_color` for styles inside the
`syntax` map for themes defined in `theme.json`. The field is optional
so there should be no backwards compatibility issues.

It is worth noting that the current behaviour for selecting text is that
the background colours will mix/blend (I'm not sure the correct term
here). Changing this behaviour, or making it configurable, looks to be a
far more complex issue and I'm not sure I know how to do it.

crates/theme_importer/src/vscode/converter.rs

@@ -241,6 +241,7 @@ impl VsCodeThemeConverter {
 
             let highlight_style = HighlightStyleContent {
                 color: token_color.settings.foreground.clone(),
+                background_color: token_color.settings.background.clone(),
                 font_style: token_color
                     .settings
                     .font_style