[WIP] ES modules

[goto-bus-stop]

Adds node-style experimental ES modules support. pass esm: true to interpret .mjs files as ES modules.

ES modules get a rec.esm property so that later parts in the browserify pipeline can identify them. rec.esm is an object containing properties:

• rec.esm.imports - array of imported bindings
• rec.esm.exports - array of exported bindings

Imported bindings are objects of the form:

• binding.from - source module (value "a" in import "a")
• binding.import - imported binding
• binding.as - local name for the binding (often the same as binding.import)
• binding.esm - true if the source module is also an ES module, false if it is a commonjs module.

Exported bindings are objects of the form:

• binding.export - local name of the binding (can be undefined if export default-ing an unnamed expression)
• binding.as - public name of the binding

Transforms can check opts._flags.esm to see if the module being transformed is an ES module (true) or a CommonJS module (false). It might be useful if transforms could also set opts._flags.esm to mark their output as an ES module, for languages that compile to ES modules.

[arlac77]

@goto-bus-stop any plans to continue on this PR ?