Unify `AST::ExternalFunctionitem` with `AST::Function`

[P-E-P]

The interface for both types is almost the same as well as the attributes. Since #2755 function body can now be optional. This means an ExternalFunctionItem could technically be replaced with a function.

One grey area remains: External function arguments cannot be pattern (unless it is a named variadic argument).

[0xn4utilus]

Hello, Is anyone currently working on this issue? I want to work on this issue

[CohenArthur]

Closed in #2849