How to debug v8 hello world sample on vscode?

[Feng Yu]

I tried, but fail, below is how I do:

1. build
```

tools/dev/gm.py x64.debug

```
2. make a launch.json file
```

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "v8 sample Debug",
      "type": "cppdbg",
      "request": "launch",
      "targetArchitecture": "x64",
      "program": "${workspaceFolder}/out/x64.debug/v8_hello_world",
      "args": [], // Optional command line args
      "stopAtEntry": true,
      "cwd": "${workspaceFolder}",
      "MIMode": "lldb"
    }
  ]
}
```
3. run and debug "v8 sample Debug"

[Feng Yu]

I forget leave a step to build hello world sample: 

```

ninja -C out/x64.debug v8_hello_world
```