docs(configuration): add output.environment.asyncFunction (#7122)

Author: snitin315

src/content/configuration/output.mdx

@@ -452,6 +452,8 @@ module.exports = {
     environment: {
       // The environment supports arrow functions ('() => { ... }').
       arrowFunction: true,
+      // The environment supports async function and await ('async function () { await ... }').
+      asyncFunction: true,
       // The environment supports BigInt as literal (123n).
       bigIntLiteral: false,
       // The environment supports const and let for variable declarations.