refactor: introduce syntax/expression package

[eddycharly]

Explanation

Introduce syntax/expression package.

[codecov]

Codecov Report

Attention: Patch coverage is 44.70588% with 47 lines in your changes missing coverage. Please review.

Project coverage is 23.03%. Comparing base (4a9eb50) to head (5732cdd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/engine/assert/parse.go	26.66%	22 Missing ⚠️
pkg/engine/assert/binding.go	0.00%	20 Missing ⚠️
pkg/engine/assert/project.go	50.00%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #491      +/-   ##
==========================================
- Coverage   23.24%   23.03%   -0.21%     
==========================================
  Files          88       88              
  Lines        4092     4090       -2     
==========================================
- Hits          951      942       -9     
- Misses       3055     3063       +8     
+ Partials       86       85       -1     

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