meta: improve typing for unpack

[lewis6991]

Fixes #1353

Not sure if this is the best way to fix this problem, but it does remove many false diagnostics from our codebase.

[sumneko]

Maybe just change it to params table and returns ...

[lewis6991]

That's what the definition above does, which fails to properly propagate types on code like:

local a, b = unpack({'a', 2})

Where:

• a should be inferred as a string
• b should be inferred as an integer.