Update the Babel plugin to remove empty static blocks

This commit updates the Babel plugin to:
- apply the same flattening logic that we already
  have for blocks, to flatten blocks nested inside
  class static blocks
- remove class static blocks when, after flattening
  all the blocks they contain, they are empty.

Before this commit, the transform output was the
same as the input.

external/builder/fixtures_babel/staticblock.js

@@ -0,0 +1,20 @@
+class A {
+  static {}
+  static {
+    { foo() }
+  }
+  static {
+    {;}
+  }
+  static {
+    if (PDFJSDev.test('TRUE')) {
+      var a = 0;
+    }
+  }
+
+  static {
+    if (PDFJSDev.test('FALSE')) {
+      var a = 1;
+    }
+  }
+}