Fix BEGIN READ WRITE regression for Redshift syntax

[divaltor]

Redshift allows BEGIN with transaction mode modifiers such as READ WRITE or ISOLATION LEVEL. Treat those tokens like transaction starters so split() does not mistake BEGIN for a procedural block.

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• ran the tests (pytest)
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Described in #843

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.52%. Comparing base (7334ac9) to head (aadda6b).
⚠️ Report is 100 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #844      +/-   ##
==========================================
+ Coverage   97.04%   97.52%   +0.47%     
==========================================
  Files          20       31      +11     
  Lines        1558     3590    +2032     
  Branches        0      314     +314     
==========================================
+ Hits         1512     3501    +1989     
- Misses         46       54       +8     
- Partials        0       35      +35     

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