added 'lexical scope' feature.

[mori0091]

No description provided.

[codecov]

Codecov Report

Merging #120 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage   94.41%   94.45%   +0.04%     
==========================================
  Files          87       87              
  Lines        1611     1624      +13     
==========================================
+ Hits         1521     1534      +13     
  Misses         90       90              

Impacted Files	Coverage Δ
example/pug-lang/include/interpreter/context.h	100.00% <100.00%> (ø)
example/pug-lang/include/interpreter/interpreter.h	97.22% <100.00%> (+0.12%)	⬆️
example/pug-lang/include/parser/stmt.h	100.00% <100.00%> (ø)
example/pug-lang/include/types/Expr.h	98.66% <100.00%> (+0.04%)	⬆️

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