feat[lang]!: make @external modifier optional in .vyi files

[cyberthirst]

What I did

• as per Panic on expected interface method implemented as internal. #4169 (comment): make @external visibility in vyi files optional
• also should fix: Panic on expected interface method implemented as internal. #4169

How to verify it

• added some tests

Commit Message

make `@external` visibility in `.vyi` files optional.

additionally, fix a panic when functions in an interface have the wrong
visibility

edit: linked wrong issue

[charles-cooper]

thanks!

[pcaversaccio]

I think the error message can be confusing since it could imply that payable is disallowed (which is not). @cyberthirst how about:

Interface functions' visibility can only be marked as `@external`

[cyberthirst]

hmm, we've improved the reporting a bit, so the message refers to the corresponding source construct

@internal
@payable
def bar():
    ...

would yield:

vyper.exceptions.FunctionDeclarationException: Interface functions can only be marked as `@external`

  contract "tests/custom/i.vyi:1", function "bar", line 1:1 
  ---> 1 @internal
  --------^
       2 @payable

would you say that it's still confusing?

[pcaversaccio]

I personally think error messages should be as precise as possible and thus I still think we should mention somehow that it's a visibility decorator.