fix[lang]: fix .vyi function body check

[cyberthirst]

What I did

• make interface body stricter to only allow exprs
• fix Un-precise Interface Body Check #4122

How to verify it

• added both a failing and passing tests

commit message

previously the validation for function bodies in interfaces incorrectly
allowed other types of expressions with the `.value` member, e.g.

```vyper
@external
def some_interface_function():
    return ...
```
although the body should only be allowed to be an ellipsis expr (`...`).

this commit correctly rejects the above source code

[charles-cooper]

once we have refined the type of body[0], we don't need to use get(), we can just directly reference .value.

[cyberthirst]

addressed in: f006571

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 46.19%. Comparing base (a1af967) to head (f006571).
Report is 54 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/semantics/types/function.py	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (a1af967) and HEAD (f006571). Click for more details.

HEAD has 139 uploads less than BASE

Flag	BASE (a1af967)	HEAD (f006571)
	140	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4177       +/-   ##
===========================================
- Coverage   91.35%   46.19%   -45.16%     
===========================================
  Files         109      109               
  Lines       15613    15614        +1     
  Branches     3436     3436               
===========================================
- Hits        14263     7213     -7050     
- Misses        920     7847     +6927     
- Partials      430      554      +124     

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