MsSql: Disallow unquoted FROM keyword as alias assignment expression

[LucaCappelletti94]

In dialects supporting = alias assignment (T-SQL/MsSql), SELECT a = FROM previously parsed as an alias assignment (SelectItem::ExprWithAlias) with FROM as the expression. This displayed as SELECT FROM AS a, which fails to re-parse because unquoted FROM is not a valid expression in a select item list. Now the round trip works as expected.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.10%. Comparing base (bfc7d38) to head (837dc7a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2546   +/-   ##
=======================================
  Coverage   81.09%   81.10%           
=======================================
  Files          42       42           
  Lines       33682    33687    +5     
  Branches    33682    33687    +5     
=======================================
+ Hits        27316    27321    +5     
  Misses       2796     2796           
  Partials     3570     3570           

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