Parse the DEFERRABLE transaction mode

[LucaCappelletti94]

This PR adds support for the deferrable mode, which is the third transaction mode of the SQL standard and of PostgreSQL.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.15%. Comparing base (bfc7d38) to head (5361adf).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ast/mod.rs	0.00%	1 Missing ⚠️
src/parser/mod.rs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2505      +/-   ##
==========================================
+ Coverage   81.09%   81.15%   +0.05%     
==========================================
  Files          42       42              
  Lines       33682    33747      +65     
  Branches    33682    33747      +65     
==========================================
+ Hits        27316    27387      +71     
- Misses       2796     2797       +1     
+ Partials     3570     3563       -7     

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