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/_utilities.scss

@@ -514,32 +514,55 @@ $utilities: map-merge(
     "color": (
       property: color,
       class: text,
+      local-vars: (
+        "text-opacity": 1
+      ),
       values: map-merge(
-        $theme-colors,
+        $utilities-text-colors,
         (
-          "white": $white,
-          "body": $body-color,
           "muted": $text-muted,
-          "black-50": rgba($black, .5),
-          "white-50": rgba($white, .5),
+          "black-50": rgba($black, .5), // deprecated
+          "white-50": rgba($white, .5), // deprecated
           "reset": inherit,
         )
       )
     ),
+    "text-opacity": (
+      css-var: true,
+      class: text-opacity,
+      values: (
+        25: .25,
+        50: .5,
+        75: .75,
+        100: 1
+      )
+    ),
     // scss-docs-end utils-color
     // scss-docs-start utils-bg-color
     "background-color": (
       property: background-color,
       class: bg,
+      local-vars: (
+        "bg-opacity": 1
+      ),
       values: map-merge(
-        $theme-colors,
+        $utilities-bg-colors,
         (
-          "body": $body-bg,
-          "white": $white,
           "transparent": transparent
         )
       )
     ),
+    "bg-opacity": (
+      css-var: true,
+      class: bg-opacity,
+      values: (
+        10: .1,
+        25: .25,
+        50: .5,
+        75: .75,
+        100: 1
+      )
+    ),
     // scss-docs-end utils-bg-color
     "gradient": (
       property: background-image,