Support declaration merging for global records

[farism]

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

-- 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.

[hishamhm]

Allowing for the extension of global records would be handy

I'm not sure we'd like to have that happen automatically, but it's something to think about!

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

If it doesn't flag an inconsistency, that's probably a bug!