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/meta_commands.rs

@@ -26,13 +26,13 @@ impl MetaCommand {
         match self {
             MetaCommand::Exit => MetaCommandExecuteResult { should_exit: true },
             MetaCommand::About => {
-                        print!("{}", branding::about_msg());
-                        MetaCommandExecuteResult { should_exit: false }
-                    }
+                print!("{}", branding::about_msg());
+                MetaCommandExecuteResult { should_exit: false }
+            }
             MetaCommand::Version => {
                 print!("{}", branding::version_msg());
-                        MetaCommandExecuteResult { should_exit: false }
-            },
+                MetaCommandExecuteResult { should_exit: false }
+            }
         }
     }
 }