Allow transpiled JS to be imported by external modules

[lavaleri]

Currently the Dafny to JS transpiler compiles JS that is very difficult to reference from an external module. As is, there are a couple of issues that block being able to use generated code in a meaningful way:

• In an external module one is unable to either import or require objects defined in the generated JS. For my own work I have been getting around this by manually inserting module.exports = { ... } in the generated file to explicitly export any module/class/object that I need to reference in my external module.
• The generated file, if imported as-is, fails to compile if it contains externs. This is because the generated file references things that must first be defined in extern files. In my own work, I get around this by manually adding const myExtern = require(./path/to/extern) to the generated javascript.
• If your externs must refer to objects defined in the generated JS, there is no clean way to resolve circular dependencies. This is because the dafny to javascript transpiler compiles a set of dafny files into a single generated JS file. The generated JS contains a module that depends on definitions in extern files, which then prevents your extern files from depending on any other module in the generated JS.

Additionally, we need documentation to show how one can build a JS module using the dafny to JS transpiler, which looks like any other native module, and can be imported and used the same as any other native module.