Replace the webpack+acorn transform with a Babel plugin

This commit converts the pdfjsdev-loader transform into a Babel plugin,
to skip a AST->string->AST round-trip.

Before this commit, the webpack build process was:
1. Babel parses the code
2. Babel transforms the AST
3. Babel generates the code
4. Acorn parses the code
5. pdfjsdev-loader transforms the AST
6. @javascript-obfuscator/escodegen generates the code
7. Webpack parses the file
8. Webpack concatenates the files

After this commit, it is reduced to:
1. Babel parses the code
2. Babel transforms the AST
3. babel-plugin-pdfjs-preprocessor transforms the AST
4. Babel generates the code
5. Webpack parses the file
6. Webpack concatenates the files

This change improves the build time by ~25% (tested on MacBook Air M2):
- `gulp lib`: 3.4s to 2.6s
- `gulp dist`: 36s to 29s
- `gulp generic`: 5.5s to 4.0s
- `gulp mozcentral`: 4.7s to 3.2s

The new Babel plugin doesn't support the `saveComments` option of
pdfjsdev-loader, and it just always discards comments. Even though
pdfjsdev-loader supported multiple values for that option, it was
effectively ignored due to `acorn` dropping comments by default.

external/builder/fixtures_esprima/deadcode-expected.js

@@ -1,13 +1,21 @@
-function f1() {
-}
+function f1() {}
 function f2() {
- return 1;
+  return 1;
 }
 function f3() {
- var i = 0;
- throw "test";
+  var i = 0;
+  throw "test";
 }
 function f4() {
- var i = 0;
+  var i = 0;
 }
-
+var obj = {
+  method1() {},
+  method2() {}
+};
+class C {
+  method1() {}
+  method2() {}
+}
+var arrow1 = () => {};
+var arrow2 = () => {};