chore: migrate lark grammar

[tserg]

What I did

Partial implementation of #2512 to migrate Lark grammar to main directory, without exposing new AST output format.

How I did it

Replicate the earlier PR but skip exposing the AST as output.

How to verify it

See tests.

Commit message

chore: migrate lark grammar

Migrate Lark grammar to main directory to
facilitate testing

Description for the changelog

Migrate Lark grammar to main directory

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[charles-cooper]

took a quick look through, looks nice. will check more carefully later this weekend or on monday

[codecov-commenter]

Codecov Report

Merging #3082 (9c8bce4) into master (0045b71) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3082      +/-   ##
==========================================
+ Coverage   88.34%   88.36%   +0.02%     
==========================================
  Files          97       98       +1     
  Lines       11001    11020      +19     
  Branches     2602     2605       +3     
==========================================
+ Hits         9719     9738      +19     
  Misses        832      832              
  Partials      450      450              

Impacted Files	Coverage Δ
vyper/ast/grammar.py	100.00% <100.00%> (ø)

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[charles-cooper]

i think this PR needs this line to ensure the grammar is packaged with vyper: https://github.com/vyperlang/vyper/pull/2512/files#diff-60f61ab7a8d1910d86d9fda2261620314edcae5894d5aaa236b821c7256badd7R121