feat($core): functional siteConfig.evergreen (#1489)

Author: shigma

packages/@vuepress/core/lib/node/webpack/createBaseConfig.js

@@ -127,7 +127,10 @@ module.exports = function createBaseConfig (context, isServer) {
       .loader('pug-plain-loader')
       .end()
 
-  if (!siteConfig.evergreen) {
+  const evergreen = typeof siteConfig.evergreen === 'function'
+    ? siteConfig.evergreen()
+    : siteConfig.evergreen
+  if (!evergreen) {
     const libDir = path.join(__dirname, '..')
     config.module
       .rule('js')

packages/docs/docs/config/README.md

@@ -356,7 +356,7 @@ module.exports = {
 
 ### evergreen
 
-- Type: `boolean`
+- Type: `boolean | Function`
 - Default: `false`
 
 Set to `true` if you are only targeting evergreen browsers. This will disable ES5 transpilation and polyfills for IE, and result in faster builds and smaller files.

packages/docs/docs/zh/config/README.md

@@ -348,7 +348,7 @@ module.exports = {
 
 ### evergreen
 
-- 类型: `boolean`
+- 类型: `boolean | Function`
 - 默认值: `false`
 
 如果你的对象只有那些 “常青树” 浏览器，你可以将其设置成 `true`，这将会禁止 ESNext 到 ES5 的转译以及对 IE 的 polyfills，同时会带来更快的构建速度和更小的文件体积。