fix: adjust FuncionDef offset to not include decorators

[iamdefinitelyahuman]

What I did

Adjust FunctionDef offset to not include decorators. This ensures a more readable annotation when an exception is raised.

Before:

vyper.exceptions.FunctionDeclarationException: Missing or unmatched return statements in function 'foo'
line 12:0 
     11
---> 12 @external
--------^
     13 @payable

After:

vyper.exceptions.FunctionDeclarationException: Missing or unmatched return statements in function 'foo'
line 14:0 
     13 @payable
---> 14 def foo() -> uint256:
--------^
     15     pass

How I did it

When converting from Python to Vyper AST, modify FunctionDef.lineno to be +1 from the end_lineno of the final decorator.

How to verify it

Run tests.

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2102 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2102   +/-   ##
=======================================
  Coverage   85.00%   85.00%           
=======================================
  Files          83       83           
  Lines        8247     8251    +4     
  Branches     1990     1991    +1     
=======================================
+ Hits         7010     7014    +4     
  Misses        740      740           
  Partials      497      497           

Impacted Files	Coverage Δ
vyper/ast/nodes.py	93.70% <100.00%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0e6141a...81cc458. Read the comment docs.

[fubuloubu]

What about if someone puts a comment between the last decorator and the function def?

[iamdefinitelyahuman]

The only way to be more accurate is to look for the specific def token. Doable, but also a fair bit of work and this fix should solve it in 99% of cases.