remove optional chaining

This breaks on Node versions lower than version 13. Normally we
transpile everything, but we currently don't do this for the nesting
plugin since it is not really part of the `src` folder.

Will get this fixed in a better way, but for now this is a quick fix to
get everything working again!

Author: RobinMalfait

nesting/plugin.js

@@ -16,7 +16,7 @@ module.exports = function nesting(opts = postcssNested) {
     let plugin = (() => {
       if (
         typeof opts === 'function' ||
-        (typeof opts === 'object' && opts?.hasOwnProperty('postcssPlugin'))
+        (typeof opts === 'object' && opts.hasOwnProperty('postcssPlugin'))
       ) {
         return opts
       }