fix: escape backslash (#787)

packages/@vuepress/core/lib/internal-plugins/importAsyncComponent.js

 function genImportAsyncComponentFile (pages) {
   return `export function loadComponent (key) {
   switch (key) {
-${pages.map(({ key, _filePath }) => `    case "${key}": return import("${_filePath}");`).join('\n')}
+${pages.map(({ key, _filePath }) => `    case ${JSON.stringify(key)}: return import(${JSON.stringify(_filePath)});`).join('\n')}
   }
 }`
 }

packages/@vuepress/core/lib/plugin-api/option/EnhanceAppFilesOption.js

@@ -51,7 +51,7 @@ module.exports = class EnhanceAppFilesOption extends Option {
         if (fs.existsSync(filePath)) {
           destPath = await context.writeTemp(
             `app-enhancers/enhancer-${moduleId++}.js`,
-            `export { default } from '${(filePath)}'`
+            `export { default } from ${JSON.stringify(filePath)}`
           )
         } else {
           logger.debug(