Extract ASTNode::SQLSelect to a separate struct (1/5)

This will allow re-using it for SQLStatement in a later commit.

(Also split the new struct into a separate file, other query-related
types will be moved here in a follow-up commit.)

src/sqlparser.rs

@@ -1142,7 +1142,7 @@ impl Parser {
                 next_token
             ))
         } else {
-            Ok(ASTNode::SQLSelect {
+            Ok(ASTNode::SQLSelect(SQLSelect {
                 projection,
                 selection,
                 relation,
@@ -1151,7 +1151,7 @@ impl Parser {
                 order_by,
                 group_by,
                 having,
-            })
+            }))
         }
     }