refactor(CSS): move box-sizing to core/css.ts (#9201)

Apply box-sizing to all of Blockly (and thereby obviate the need
to apply it to .blocklyHtmlInput in particular.

core/toolbox/toolbox.ts

@@ -1172,6 +1172,7 @@ Css.register(`
 
 /* Category tree in Toolbox. */
 .blocklyToolbox {
+  box-sizing: border-box;
   user-select: none;
   -ms-user-select: none;
   -webkit-user-select: none;