feat: Implement Chapter 6, Parsing Expressions

[danvk]

Implementation of Chapter 6: Parsing Expressions.

I mostly followed the book implementation, except that I didn't use a class this time.

[github-actions]

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[codecov]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (2fb673d) 88.79% compared to head (27066d0) 87.32%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/parser.ts	86.30%	23 Missing ⚠️
src/main.ts	47.05%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   88.79%   87.32%   -1.48%     
==========================================
  Files           6        8       +2     
  Lines         366      568     +202     
  Branches       48       73      +25     
==========================================
+ Hits          325      496     +171     
- Misses         40       71      +31     
  Partials        1        1              

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