Update color and background-color utilities

- Adds new functions to generate additional Sass maps
- Adds new root variables for rgb() versions of our theme colors, plus a few extras
- Adds ability to change the alpha transparency of text color and background color utilities with new utilities, inline styles, or local CSS var
- Updates documentation for color and background-color utilities pages
- Deprecates .text-black-50 and .text-white-50 since those (and additional transparency levels) can be generated on the fly

Change approach slightly to prevent cascade

scss/_functions.scss

@@ -32,6 +32,41 @@
   }
 }
 
+// Colors
+@function to-rgb($value) {
+  @return red($value), green($value), blue($value);
+}
+
+@function rgba-css-var($identifier, $target) {
+  @return rgba(var(--#{$variable-prefix}#{$identifier}-rgb), var(--#{$variable-prefix}#{$target}-opacity));
+}
+
+// stylelint-disable scss/dollar-variable-pattern
+@function map-loop($map, $func, $args...) {
+  $_map: ();
+
+  @each $key, $value in $map {
+    // allow to pass the $key and $value of the map as an function argument
+    $_args: ();
+    @each $arg in $args {
+      $_args: append($_args, if($arg == "$key", $key, if($arg == "$value", $value, $arg)));
+    }
+
+    $_map: map-merge($_map, ($key: call(get-function($func), $_args...)));
+  }
+
+  @return $_map;
+}
+// stylelint-enable scss/dollar-variable-pattern
+
+@function varify($list) {
+  $result: null;
+  @each $entry in $list {
+    $result: append($result, var(--#{$variable-prefix}#{$entry}), space);
+  }
+  @return $result;
+}
+
 // Internal Bootstrap function to turn maps into its negative variant.
 // It prefixes the keys with `n` and makes the value negative.
 @function negativify-map($map) {