Postgres: Support SUBSTRING SIMILAR/ESCAPE syntax and reversed FOR/FROM order

[BenSatori]

Fixes parsing of two legal Postgres SUBSTRING forms:

• SUBSTRING('hello world' FOR 5 FROM 1) (FOR before FROM)
• SUBSTRING('hello world' SIMILAR '%#"o w#"%' ESCAPE '#')

See https://www.postgresql.org/docs/current/functions-string.html

Both previously failed with a parser error. Adds a similar flag to Expr::Substring to track the SIMILAR ... ESCAPE display form, and allows FOR to be parsed before FROM (normalized to FROM ... FOR on output).

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.09%. Comparing base (5cad906) to head (ebab908).

Files with missing lines	Patch %	Lines
src/parser/mod.rs	72.00%	0 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2494      +/-   ##
==========================================
- Coverage   81.09%   81.09%   -0.01%     
==========================================
  Files          42       42              
  Lines       33684    33719      +35     
  Branches    33684    33719      +35     
==========================================
+ Hits        27316    27344      +28     
  Misses       2798     2798              
- Partials     3570     3577       +7     

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