build: add --keep-names option to build command

[HaydenOrz]

To Resoved https://github.com/mike-lischke/antlr4ng/issues/38

What's changed

Enabling --keep-names option will change the bundle, as shown below:

1. Two new lines of code have been added to the file header, which are used to keep the name of the class or function

   var __defProp = Object.defineProperty;
   var __name = (target, value) => __defProp(target, "name", { value, configurable: true });

2. A static methods will be added to all classes(in the case of ParseTreeWalker):

   var ParseTreeWalker = class _ParseTreeWalker {
     static {
       __name(this, "ParseTreeWalker");
     }
     // ...
    }