ui.virtual.jump-label bg styling should always be rendered on top of other bgs

[RLHerbert]

Right now the bg style for "ui.virtual.jump-label" is not rendered on top causing the two-characters to get lost when, e.g., the line is highlighted underneath and "ui.virtual.jump-label" fg is the same color as the highlight.

Example with "ui.selection.primary" = { bg = "primary_highlight" } and "ui.virtual.jump-label" = {fg primary_highlight, bg = "opal" }. The jump labels cannot be seen on the highlighted line but can be seen elsewhere.

I believe "ui.virtual.jump-label.bg" should be rendered on top of all other bgs.