Fix transform of unary expression in Babel plugin

All of our static evaluation & dead-code elimination transforms need to
happen in post-order, transforming inner nodes first. This is so that
in complex nested cases all transforms see the simplified version of
their inner nodes.

For example:
  async getNimbusExperimentData() {
    if (!PDFJSDev.test("GECKOVIEW")) { return null; }
    // other code
  }
-> [evaluation of PDFJSDev.*]
  async getNimbusExperimentData() {
    if (!false) { return null; }
    // other code
  }
-> [!false -> true]
  async getNimbusExperimentData() {
    if (true) { return null; }
    // other code
  }
-> [if (true) -> replace with the if branch]
  async getNimbusExperimentData() {
    return null;
    // other code
  }
-> [early return -> remove dead code]
  async getNimbusExperimentData() {
    return null;
    // other code
  }

This was done correctly in all cases except for our `UnaryExpression`
transform, which was happening in pre-order.

web/firefoxcom.js

@@ -400,7 +400,7 @@ class ExternalServices extends BaseExternalServices {
   }
 
   async getNimbusExperimentData() {
-    if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("GECKOVIEW")) {
+    if (!PDFJSDev.test("GECKOVIEW")) {
       return null;
     }
     const nimbusData = await FirefoxCom.requestAsync(