environment: Add Math.clamp

The good old clamp function, now part of the Math family.
Clamp is happy after so many years of loneliness.

This is a strict implementation of the draft ECMAScript
proposal.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1295

js/ui/environment.js

@@ -362,6 +362,10 @@ function init() {
     const Format = imports.format;
 
     String.prototype.format = Format.format;
+
+    Math.clamp = function (x, lower, upper) {
+        return Math.min(Math.max(x, lower), upper);
+    };
 }
 
 // adjustAnimationTime: