Parse SQLite's IS and IS NOT operators

[LucaCappelletti94]

This PR adds support for SQLite accepts IS and IS NOT between arbitrary expressions. Only SQLite supports them, as far as I can tell.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.24%. Comparing base (dcbc923) to head (2d8ba04).

Files with missing lines	Patch %	Lines
src/parser/mod.rs	93.10%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2470   +/-   ##
=======================================
  Coverage   81.23%   81.24%           
=======================================
  Files          42       42           
  Lines       33891    33919   +28     
  Branches    33891    33919   +28     
=======================================
+ Hits        27533    27559   +26     
  Misses       2793     2793           
- Partials     3565     3567    +2     

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[sabir-akhadov-localstack]

One non-blocking comment inline.

[sabir-akhadov-localstack]

SQLite puts < <= > >= one level above =/IS (lang_expr.html, operator table), so a IS b < c is a IS (b < c) there while this yields (a IS b) < c; sqlite 3.46: SELECT 0 IS 1 < 2 -> 0. Same pre-existing lumping the crate does for a = b < c, so fine to leave, but the comment could say "same level as =" and link lang_expr.html rather than syntax/expr.html, which has no precedence info.

[LucaCappelletti94]

The comment describes IS and IS NOT at the same precedence level as = and links SQLite’s operator table. The preexisting comparison-precedence limitation is handled separately in #2678.