chore[ci]: update coverage exclusion rules

[charles-cooper]

improve coverage by updating coverage exclusion rules to exclude statements/branches we don't expect to hit at runtime (like panics or type checking)

What I did

How I did it

How to verify it

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

Cute Animal Picture

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.80%. Comparing base (762eec6) to head (5dd4861).

Files with missing lines	Patch %	Lines
vyper/codegen/core.py	93.33%	1 Missing ⚠️
vyper/codegen/ir_node.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4459      +/-   ##
==========================================
+ Coverage   92.09%   92.80%   +0.70%     
==========================================
  Files         119      119              
  Lines       16931    16765     -166     
  Branches     2865     2789      -76     
==========================================
- Hits        15593    15559      -34     
+ Misses        919      841      -78     
+ Partials      419      365      -54     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.