Add brightened nord3 color

The value is calculated using the Sass `lighten` (1) function to ensure
full interoperability with other port projects that providing this
theme feature.

References:
  (1) https://sass-lang.com/documentation/functions/color#lighten

GH-10

src/modules/_colors.scss

@@ -0,0 +1 @@
+$nord3-brightened: lighten($nord3, 10%);

src/nord.scss

@@ -13,6 +13,7 @@
 @import "../node_modules/nord/src/sass/nord";
 
 @import "modules/base";
+@import "modules/colors";
 @import "modules/css";
 @import "modules/diff";
 @import "modules/general";