Add converting codepoints to characters

[mat-1]

This PR adds the ability to do expressions like "97 to character". I think this makes sense since you can already do the inverse conversion of characters to codepoints.

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (bb43194) 83.27% compared to head (5c48b97) 83.20%.

Files	Patch %	Lines
core/src/ast.rs	8.33%	11 Missing ⚠️
core/src/error.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #262      +/-   ##
==========================================
- Coverage   83.27%   83.20%   -0.08%     
==========================================
  Files          52       52              
  Lines       14518    14532      +14     
==========================================
+ Hits        12090    12091       +1     
- Misses       2428     2441      +13     

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[printfn]

Thanks!