Fix ALTER TABLE grammar to use Columnkw token for COLUMN keyword

TokenType.Column (168) is an internal SQLite bytecode token, while
TokenType.Columnkw (61) is the actual keyword token produced by the
tokenizer for the COLUMN keyword. This fixes parsing of ALTER TABLE
... ALTER COLUMN ... SET NOT NULL and DROP NOT NULL statements.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: electroly

src/SqlNotebookScript/Interpreter/SqliteGrammar.cs

@@ -130,7 +130,7 @@ static SqliteGrammar()
                 ),
                 Prod($"{p}.alter", 1,
                     Tok(TokenType.Alter),
-                    Opt(Tok(TokenType.Column)),
+                    Opt(Tok(TokenType.Columnkw)),
                     Id("column name"),
                     Or(
                         Prod($"{p}.alter-set", 1,