Update ionRangeSlider to use CSS variables in a Bootstrap 5 context

[gadenbuie]

ionRangeSlider's sass still uses color functions that result in realized colors -- as opposed to CSS variables -- once compiled.

shiny/inst/www/shared/ionrangeslider/scss/shiny.scss

Lines 39 to 46 in e2b7f91

	$bg: $body-bg !default;
	$fg: color-contrast($body-bg) !default;
	$accent: #428bca !default;
	// "Low-level" coloring, borders, and fonts
	$line_bg: linear-gradient(to bottom, mix($bg, $fg, 87%) -50%, $bg 150%) !default;
	$line_bg_color: mix($bg, $fg, 93%) !default;
	$line_border: 1px solid mix($bg, $fg, 80%) !default;

We should either:

1. Update ionRangeSlider/scss/shiny.scss to avoid these color calculations.
2. Add ionRangeSlider/scss/bootstrap5.scss to avoid these color calculations in BS5, renaming the current shiny.scss to bootstrap4.scss.