SQLite, DuckDB, PostgreSQL: Accept ._name after whitespace in a qualified name

[LucaCappelletti94]

SELECT t ._a FROM t failed to tokenize with Unexpected character '_', although SQLite 3.51.1, DuckDB 1.4.4 and PostgreSQL 16 all run it and read it as t._a. The tokenizer treats ._ as a period only when the previous token is a word, and with whitespace in between the previous token was the whitespace, so it fell through to number parsing.

The check now looks at the previous token that is not whitespace. A ._ that follows a number, as in SELECT 1 ._a, is still a tokenizer error, and SELECT ._abc with no qualifier is still rejected by the parser. Tests cover these forms in every dialect where _ can start an identifier.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.32%. Comparing base (ada8314) to head (32a2953).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tokenizer.rs	97.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2677      +/-   ##
==========================================
+ Coverage   81.28%   81.32%   +0.04%     
==========================================
  Files          42       42              
  Lines       33920    33980      +60     
  Branches    33920    33980      +60     
==========================================
+ Hits        27571    27634      +63     
+ Misses       2785     2784       -1     
+ Partials     3564     3562       -2     

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