Debugging your task code in dev is supported via VS Code, without having to pass in any additional flags. Create a launch configuration in .vscode/launch.json:
launch.json
{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Trigger.dev: Dev",
      "type": "node",
      "request": "launch",
      "cwd": "${workspaceFolder}",
      "runtimeExecutable": "npx",
      "runtimeArgs": ["trigger.dev@latest", "dev"],
      "skipFiles": ["<node_internals>/**"],
      "sourceMaps": true
    }
  ]
}
Then you can start debugging your tasks code by selecting the Trigger.dev: Dev configuration in the debug panel, and set breakpoints in your tasks code.