feat: introduce acorn as parser

[antfu]

fix #304

This allows users to opt-in a more accurate (but less tolerant and performant) mode.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (623073c) 98.19% compared to head (45ed965) 98.72%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/context.ts	95.78%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #307      +/-   ##
==========================================
+ Coverage   98.19%   98.72%   +0.53%     
==========================================
  Files          10       14       +4     
  Lines        1440     1811     +371     
  Branches      288      368      +80     
==========================================
+ Hits         1414     1788     +374     
+ Misses         26       23       -3     

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[antfu]

Shipping this as it's an opt-in feature that we could iterate faster