Add LOWER builtin function

[agonist]

Adds LOWER to builtins functions to format TEXT fields to lower-case.

For non TEXT fields it will return NULL.

Types that can be casted to TEXT will work if they're first explicitly casted.

[asdine]

Great job @agonist !

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (f94c703) 79.09% compared to head (b00e4ea) 79.04%.

❗ Current head b00e4ea differs from pull request most recent head 53799fe. Consider uploading reports for the commit 53799fe to get more accurate results

Files	Patch %	Lines
internal/expr/functions/builtins.go	57.14%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #510      +/-   ##
==========================================
- Coverage   79.09%   79.04%   -0.05%     
==========================================
  Files         125      125              
  Lines       10795    10816      +21     
==========================================
+ Hits         8538     8550      +12     
- Misses       1541     1549       +8     
- Partials      716      717       +1     

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