refactor[parser]: refactor pragma parsing

[charles-cooper]

move pragma parsing into its own subroutine

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

[cyberthirst]

approved without review

[codecov]

Codecov Report

Attention: Patch coverage is 87.87879% with 4 lines in your changes missing coverage. Please review.

Project coverage is 92.27%. Comparing base (94cf162) to head (61b5715).

Files with missing lines	Patch %	Lines
vyper/ast/pre_parser.py	87.87%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4530   +/-   ##
=======================================
  Coverage   92.27%   92.27%           
=======================================
  Files         123      123           
  Lines       17520    17522    +2     
  Branches     2961     2961           
=======================================
+ Hits        16166    16168    +2     
  Misses        946      946           
  Partials      408      408           

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

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.