feat: allow global declarations in modules

[crawford]

TypeScript allows declarations in the global namespace from within
ambient module declarations. This form is used in several of the
declaration files for Node.js (e.g. buffer.d.ts, console.d.ts).

[amaanq]

Hey, thanks for the PR! After taking a look at the TS handbook and AST output, I think a better name might be global_module, what do you think? Also, throw it under internal_module to group it a little more nicely, and rebase if you can, I just pushed some changes

[crawford]

@amaanq I agree, global_module makes more sense. And I'll rebase your changes. I want to make sure I understand your comment about internal_module though. How do you expect the following snippet to parse?

declare module "foo" {
  global {
  }
}

Like this?

...
      (statement_block
        (expression_statement
          (internal_module
            (identifier)
            (statement_block
...

Or like this?

...
      (statement_block
        (expression_statement
          (global_module
            (statement_block
...

I experimented with a few different options but I noticed ambiguity with the existing support for global namespace declarations. The following isn't understood to be a global module currently:

declare global {
}

It just parses as follows:

(program
  (ambient_declaration
    (statement_block)))

Is it going to cause downstream trouble if we change this to parse as something like this?

(program
  (ambient_declaration
    (global_module
      (statement_block))))

Ultimately, what I'm trying to do is identify objects defined in the global namespace so I can track them separately.