feat[lang]!: add feature flag for decimals

[charles-cooper]

What I did

depends #3929

How I did it

How to verify it

Commit message

hide decimals behind a feature flag. this informally lets us "break the
compatibility contract" with users by moving decimals into
quasi-experimental status; this way we can iterate on the decimals
design faster in the 0.4.x series instead of needing to wait for
breaking releases to make changes.

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 68.00000% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 90.83%. Comparing base (e4f1c24) to head (ba8868a).

Files	Patch %	Lines
vyper/ast/pre_parser.py	0.00%	3 Missing and 1 partial ⚠️
vyper/cli/vyper_compile.py	33.33%	1 Missing and 1 partial ⚠️
vyper/compiler/settings.py	66.66%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3930      +/-   ##
==========================================
- Coverage   90.89%   90.83%   -0.06%     
==========================================
  Files          95       95              
  Lines       14393    14414      +21     
  Branches     3190     3196       +6     
==========================================
+ Hits        13082    13093      +11     
- Misses        908      915       +7     
- Partials      403      406       +3     

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