improve performance of getIconSize() (#10125)

* load icons once on module load

lib/svg-helpers.js

 import SVGPathCommander from 'svg-path-commander'
 import loadSimpleIcons from './load-simple-icons.js'
 
+const simpleIcons = loadSimpleIcons()
+
 function svg2base64(svg) {
   return `data:image/svg+xml;base64,${Buffer.from(svg.trim()).toString(
     'base64',
@@ -8,8 +10,6 @@ function svg2base64(svg) {
 }
 
 function getIconSize(iconKey) {
-  const simpleIcons = loadSimpleIcons()
-
   if (!(iconKey in simpleIcons)) {
     return undefined
   }