Easy merging of new grid breakpoints and containers (#26714)

This change follows exactly what was done for the `$grays`, `$colors`, and `$theme-colors` lists in #23260. This allows for easy addition of new breakpoints or grid containers also.

scss/_variables.scss

@@ -184,13 +184,18 @@ $paragraph-margin-bottom:   1rem !default;
 // Define the minimum dimensions at which your layout will change,
 // adapting to different screen sizes, for use in media queries.
 
-$grid-breakpoints: (
-  xs: 0,
-  sm: 576px,
-  md: 768px,
-  lg: 992px,
-  xl: 1200px
-) !default;
+$grid-breakpoints: () !default;
+// stylelint-disable-next-line scss/dollar-variable-default
+$grid-breakpoints: map-merge(
+  (
+    xs: 0,
+    sm: 576px,
+    md: 768px,
+    lg: 992px,
+    xl: 1200px
+  ),
+  $grid-breakpoints
+);
 
 @include _assert-ascending($grid-breakpoints, "$grid-breakpoints");
 @include _assert-starts-at-zero($grid-breakpoints);
@@ -200,12 +205,17 @@ $grid-breakpoints: (
 //
 // Define the maximum width of `.container` for different screen sizes.
 
-$container-max-widths: (
-  sm: 540px,
-  md: 720px,
-  lg: 960px,
-  xl: 1140px
-) !default;
+$container-max-widths: () !default;
+// stylelint-disable-next-line scss/dollar-variable-default
+$container-max-widths: map-merge(
+  (
+    sm: 540px,
+    md: 720px,
+    lg: 960px,
+    xl: 1140px
+  ),
+  $container-max-widths
+);
 
 @include _assert-ascending($container-max-widths, "$container-max-widths");