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.)

tests/sqlparser_ansi.rs

@@ -11,7 +11,7 @@ fn parse_simple_select() {
     let sql = String::from("SELECT id, fname, lname FROM customer WHERE id = 1");
     let ast = parse_sql(&sql);
     match ast {
-        ASTNode::SQLSelect { projection, .. } => {
+        ASTNode::SQLSelect(SQLSelect { projection, .. }) => {
             assert_eq!(3, projection.len());
         }
         _ => assert!(false),