refactor: avoid using regexp's lookbehind (#323)

lib/prepare.js

@@ -236,14 +236,14 @@ async function genComponentRegistrationFile ({ sourceDir }) {
   return `import Vue from 'vue'\n` + components.map(genImport).join('\n')
 }
 
-const indexRE = /(?<=(^|\/))(index|readme)\.md$/i
+const indexRE = /(^|.*\/)(index|readme)\.md$/i
 const extRE = /\.(vue|md)$/
 
 function fileToPath (file) {
   if (isIndexFile(file)) {
     // README.md -> /
     // foo/README.md -> /foo/
-    return '/' + file.replace(indexRE, '')
+    return file.replace(indexRE, '/$1')
   } else {
     // foo.md -> /foo.html
     // foo/bar.md -> /foo/bar.html