app\app.ino is hardcoded when initializing a project

[akaroml]

1. Open an empty folder named "blink" and create a new .ino file inside it
2. Select a board (any board)
3. Run Arduino: Verify

Now arduino.json is generated under .vscode folder and the sketch property is set to "app\app.ino" which does not reflect the real file name.

[akaroml]

We need to fix this before the first release @yaohaizh