Deprecate default-margin and default-padding Sass variables

Please use custom properties instead.

app/assets/stylesheets/alchemy/_deprecated_variables.scss

@@ -0,0 +1,4 @@
+@import "alchemy/deprecation";
+
+@include alchemy-deprecated-variable("$default-padding", "var(--spacing-1)");
+@include alchemy-deprecated-variable("$default-margin", "var(--spacing-1)");

app/assets/stylesheets/alchemy/_deprecation.scss

@@ -0,0 +1,17 @@
+@charset "UTF-8";
+
+/// Throws Sass warnings to announce backend deprecations. You can disable them
+/// by setting the `$output-alchemy-deprecation-warnings` variable to `false`.
+///
+/// @access private
+
+$output-alchemy-deprecation-warnings: true !default;
+
+@mixin alchemy-deprecated-variable($variable, $replacement) {
+  @if $output-alchemy-deprecation-warnings == true {
+    @if global-variable-exists($variable) {
+      @warn "[Alchemy] [Deprecation] Sass variable `#{$variable}` is deprecated and will be " +
+        "removed in Alchemy 8.0. Please use custom property `#{$replacement}` instead";
+    }
+  }
+}

app/assets/stylesheets/alchemy/_variables.scss

@@ -1,3 +1,5 @@
+@import "alchemy/deprecated_variables";
+
 $light-blue: #cddce5;
 $very-light-blue: lighten($light-blue, 12%);
 $blue: #2d5a8d;