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[bot]]

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 :warning:
core/src/error.rs	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ 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!