Improve JavaScript target code template.

[ghost]

• Added private field in package.json, to prevent someone accidentally publish the generated code into NPM repository.
• Use get-stdin-with-tty to read stdin as a string (fs.readFileSync(0,'utf-8') not works well on windows, Accessing stdin with file descriptor 0 on Windows causes exception if there's no input / if stdin is closed nodejs/node#19831).
• Use let and const instead of var.
• Move error count to a field inside MyErrorListener class

TODO:

• Wait for ANTLR version and update dependency, so we can avoid those bugs in JS runtime.
• Improve other code template.
• Use file (or whole directory) as template, so we needn't hard code them in C# code.

[kaby76]

I'm going to need to rewrite the code sooner than later to use StringTemplates. I posted some runtimes in Antlr4 issues and have asked to add in warm-up code. I don't want to just update the WriteLine implementation with yet another option switch on the command line for "performance testing", because I always anticipated this would all be throw-away code. So, I'm going to now make a branch "StringTemplates" that will contain the rewrite. I hope to rewrite that over the next couple days. At that point, it will come with some default templates, but folks can override that with their own templates.