Support declaration merging for global records

Allowing for the extension of global records would be handy, e.g.

```tl
-- some_library.tl
global record my_record
  func_a: function()
end

-- my_app.tl
global record my_record
  func_b: function()
end
```

Right now it doesn't error if a global record is defined twice, but it only picks up the first one it finds.