feat(parser): implement insert command parsing

Implements the parse_insert_command function to handle the insert
statement according to the grammar definition. The function now
correctly parses the command format insert int string string and creates
a structured Statement::Insert with the id, username, and email fields.

src/command.rs

@@ -80,7 +80,12 @@ mod tests {
 
     #[test]
     fn test_execute_insert_statement() {
-        let statement: Command = Statement::Insert.into();
+        let statement: Command = Statement::Insert {
+            id: 45,
+            username: String::from("user"),
+            email: String::from("user@example.org"),
+        }
+        .into();
         let result = statement.execute().display();
         assert_snapshot!(result);
     }