sqlite: add close method, and an is_open field (#7382)

vlib/sqlite/sqlite_test.v

@@ -4,9 +4,10 @@ fn test_sqlite() {
 	$if !linux {
 		return
 	}
-	db := sqlite.connect(':memory:') or {
+	mut db := sqlite.connect(':memory:') or {
 		panic(err)
 	}
+	assert db.is_open
 	db.exec('drop table if exists users')
 	db.exec("create table users (id integer primary key, name text default '');")
 	db.exec("insert into users (name) values ('Sam')")
@@ -25,4 +26,8 @@ fn test_sqlite() {
 		panic(err)
 	}
 	assert user.vals.len == 2
+	db.close() or {
+		panic(err)
+	}
+	assert !db.is_open
 }