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.

external/builder/babel-plugin-pdfjs-preprocessor.mjs

@@ -92,17 +92,22 @@ function babelPluginPDFJSPreprocessor(babel, ctx) {
           }
         },
       },
-      UnaryExpression(path) {
-        const { node } = path;
-        if (node.operator === "typeof" && isPDFJSPreprocessor(node.argument)) {
-          // typeof PDFJSDev => 'object'
-          path.replaceWith(t.stringLiteral("object"));
-          return;
-        }
-        if (node.operator === "!" && t.isBooleanLiteral(node.argument)) {
-          // !true => false,  !false => true
-          path.replaceWith(t.booleanLiteral(!node.argument.value));
-        }
+      UnaryExpression: {
+        exit(path) {
+          const { node } = path;
+          if (
+            node.operator === "typeof" &&
+            isPDFJSPreprocessor(node.argument)
+          ) {
+            // typeof PDFJSDev => 'object'
+            path.replaceWith(t.stringLiteral("object"));
+            return;
+          }
+          if (node.operator === "!" && t.isBooleanLiteral(node.argument)) {
+            // !true => false,  !false => true
+            path.replaceWith(t.booleanLiteral(!node.argument.value));
+          }
+        },
       },
       LogicalExpression: {
         exit(path) {