chore: using export default at config.ts

Current `target` is `es2018`, using `export =` (export assignment) will trigger `ts(1203)`<sub>1</sub> error:

    Export assignment cannot be used when targeting ECMAScript modules.
    Consider using 'export default' or another module format instead.

[1] https://github.com/microsoft/TypeScript/blob/e9868e96e87996df46a13b4323866acc639e71ce/src/compiler/diagnosticMessages.json#L634

packages/docs/docs/.vuepress/config.ts

@@ -6,7 +6,7 @@ import {
   NavItems4ZH
 } from './config/index'
 
-export = defineConfig(ctx => ({
+export default defineConfig(ctx => ({
   theme: '@vuepress/vue',
   dest: '../../vuepress',
   head: [
@@ -123,4 +123,4 @@ export = defineConfig(ctx => ({
   ],
   extraWatchFiles: ['.vuepress/config/**'],
   evergreen: !ctx.isProd
-}));
+}))