Generate border widths instead of hard-coding

Sadly we are forced to expose a global @default-border-color variable
but what can you do. Trying to match against the border-colors scale
was too insane; I either polluted the scope and broke a bunch of other
utilities or I made the border width utilities non-mix-in-able which is
also grim af.

src/generators.less

@@ -4,6 +4,7 @@
 
 @import "generators/backgrounds";
 @import "generators/border-colors";
+@import "generators/border-widths";
 @import "generators/constrain";
 @import "generators/text-colors";
 @import "generators/text-weights";