Add coalesce function

[yaziine]

This PR fixes #509 by adding support to COALESCE method.
Unlike PosgreSQL, our implementation doesn't fail if the arguments aren't of the same type.
However, an error is returned if there is no argument at all.

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (6dc2964) 78.94% compared to head (31c0128) 78.87%.
Report is 1 commits behind head on main.

❗ Current head 31c0128 differs from pull request most recent head e065575. Consider uploading reports for the commit e065575 to get more accurate results

Files	Patch %	Lines
internal/expr/functions/builtins.go	53.33%	6 Missing and 1 partial ⚠️
internal/expr/functions/definition.go	33.33%	1 Missing and 1 partial ⚠️
internal/sql/parser/parser.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #513      +/-   ##
==========================================
- Coverage   78.94%   78.87%   -0.07%     
==========================================
  Files         126      126              
  Lines       10890    10907      +17     
==========================================
+ Hits         8597     8603       +6     
- Misses       1571     1579       +8     
- Partials      722      725       +3     

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[asdine]

👏🏼