fix: make plugin pure for tree-shaking

Author: dangreen

.size-limit.cjs

@@ -7,7 +7,7 @@ function modifyWebpackConfig(config) {
 module.exports = [
   {
     path: 'dist/chart.js',
-    limit: '77.5 KB',
+    limit: '77.7 KB',
     webpack: false,
     running: false
   },

src/plugins/plugin.colors.ts

@@ -13,7 +13,7 @@ const BORDER_COLORS = [
 ];
 
 // Primary colors with 50% transparency
-const BACKGROUND_COLORS = BORDER_COLORS.map(_ => _.replace(')', ', 0.5)'));
+const BACKGROUND_COLORS = /* #__PURE__ */ BORDER_COLORS.map(_ => _.replace(')', ', 0.5)'));
 
 function getBorderColor(i: number) {
   return BORDER_COLORS[i % BORDER_COLORS.length];