PostgreSQL: Reject string literals as aliases

[LucaCappelletti94]

PostgreSQL does not accept a single-quoted string as a column or table alias. The SELECT reference defines both as identifiers (https://www.postgresql.org/docs/current/sql-select.html), and PostgreSQL 16 rejects SELECT 1 AS 'x', SELECT 1 'x', SELECT a FROM t AS 'y' and SELECT a FROM t 'y' with a syntax error. The parser accepted a string alias in every dialect.

A new dialect method, supports_string_literal_alias, defaults to true so the other dialects are unchanged, and PostgreSqlDialect sets it to false. Tests check the exact errors for column and table aliases in PostgreSQL and that every other dialect still accepts both.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.25%. Comparing base (1aabd12) to head (34037e1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2668   +/-   ##
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  Coverage   81.25%   81.25%           
=======================================
  Files          42       42           
  Lines       33910    33917    +7     
  Branches    33910    33917    +7     
=======================================
+ Hits        27553    27560    +7     
  Misses       2792     2792           
  Partials     3565     3565           

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[s5dsn-eqee]

lgtm!

[LucaCappelletti94]

Merging, thanks @s5dsn-eqee for the review!

cc @alamb @iffyio