PostgreSQL: Parse custom typed string literals

[LucaCappelletti94]

For an existing user-defined type mood, PostgreSQL permits SELECT mood 'happy', meaning the value 'happy' with type mood. The parser rejects it in projections, predicates, and function arguments.

This PR adds support for this typed-constant syntax for user-defined types.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.33%. Comparing base (d32825d) to head (0bd18e1).

Files with missing lines	Patch %	Lines
src/parser/mod.rs	97.14%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2689      +/-   ##
==========================================
+ Coverage   81.29%   81.33%   +0.03%     
==========================================
  Files          42       42              
  Lines       33950    33981      +31     
  Branches    33950    33981      +31     
==========================================
+ Hits        27601    27638      +37     
  Misses       2785     2785              
+ Partials     3564     3558       -6     

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