Add UPPER function

[agonist]

add the UPPER function.

In addition to the upper function, the PR adds a strings definitions package in prevision of adding a bunch of string related functions.

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (b034800) 79.06% compared to head (0c13325) 79.03%.

❗ Current head 0c13325 differs from pull request most recent head 2f80922. Consider uploading reports for the commit 2f80922 to get more accurate results

Files	Patch %	Lines
internal/expr/functions/strings.go	59.09%	16 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #511      +/-   ##
==========================================
- Coverage   79.06%   79.03%   -0.04%     
==========================================
  Files         125      126       +1     
  Lines       10816    10840      +24     
==========================================
+ Hits         8552     8567      +15     
- Misses       1547     1555       +8     
- Partials      717      718       +1     

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