Support Export Assignment

[evaera]

Support export assignment (export =) and handle importing with import x = require(...). This will make typing Lua modules much more versatile.

Take care to handle export merging, for example:

declare module "something" {
  function something (): void
  namespace something {
    export interface Animal {
      species: string
    }
    export const value = 5 // values are also allowed... which is bad.
  }
  export = something
}

Export assignments that are merged with a namespace can be imported with the import * as syntax:

import * as something from "something";

something()

const x = something.value // Bad, values don't make sense here in the sense of Lua
// We should throw an error when code tries to access a member from a merged namespace 

let y: something.Animal // Good, types are useful

import { Animal } from "something" // This syntax can be used to access the namespace portion only

import something = require("something") // This syntax can be used to access the function only
// This is also the only way you can import it if there is no namespace being merged into the export.