fix[tool]: keep experimentalCodegen blank in standard json input

[charles-cooper]

What I did

How I did it

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Commit message

in standard json input, `experimentalCodegen` would default
to true, creating a conflict if the user used `#pragma
experimental-codegen`. set it to `None`.

misc/refactor:
- refactor `get_settings` function.

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 46.14%. Comparing base (f909d6e) to head (cd4ae4a).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/cli/vyper_json.py	28.57%	5 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (f909d6e) and HEAD (cd4ae4a). Click for more details.

HEAD has 139 uploads less than BASE

Flag	BASE (f909d6e)	HEAD (cd4ae4a)
	140	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4216       +/-   ##
===========================================
- Coverage   91.35%   46.14%   -45.22%     
===========================================
  Files         109      109               
  Lines       15635    15637        +2     
  Branches     3443     3443               
===========================================
- Hits        14283     7215     -7068     
- Misses        920     7867     +6947     
- Partials      432      555      +123     

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