fix(nextjs): move basePath to experimental (#141)

### Description

The required `experimental` object is missing from the configuration file. This prevents experimental features such as `basePath` to be enabled.

### Changelog

**Changed**

- moves `basePath` to new `experimental` object
- update `package.json` for Node v14 support

Author: kennylam

next.config.js

@@ -4,8 +4,10 @@ const withSass = require("@zeit/next-sass");
 const rtlcss = require("rtlcss");
 
 module.exports = withSass({
+  experimental: {
+    basePath: process.env.BASE_PATH || "",
+  },
   assetPrefix: ".",
-  basePath: process.env.BASE_PATH || "",
   env: {
     ALTLANG_ROOT_PATH: process.env.ALTLANG_ROOT_PATH || "/",
     ENABLE_RTL: process.env.ENABLE_RTL || "false",