Allow SYNC keyword as implicit alias in expressions

In ClickHouse, keywords like SYNC can be used as column aliases without
AS keyword. Add SYNC to the list of allowed implicit alias keywords.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

parser/expression.go

@@ -332,7 +332,7 @@ func (p *Parser) parseImplicitAlias(expr ast.Expression) ast.Expression {
 	if !canBeAlias {
 		// Some keywords can be used as implicit aliases in ClickHouse
 		switch p.current.Token {
-		case token.KEY, token.INDEX, token.VIEW, token.DATABASE, token.TABLE:
+		case token.KEY, token.INDEX, token.VIEW, token.DATABASE, token.TABLE, token.SYNC:
 			canBeAlias = true
 		}
 	}

parser/testdata/03640_load_marks_synchronously/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt8": true
-  }
-}
+{}