Open the app with JSON content selected

When you open the app, the previous JSON you used is still there and the cursor is focussed inside the editor. However, I often find myself pasting the JSON i'd like to format before the JSON that was already there, because the JSON isn't selected yet.

Right now, the flow of formatting a JSON structure is:

1. Open json.pizza
2. Select all (CMD+A)
3. Delete (backspace)
4. Paste new JSON (CMD+V)
5. Hit format (CMD+Enter)

I'd argue that the flow should be
1. Open json.pizza
2. Paste new JSON (CMD+V)
3. Hit format (CMD+Enter)

We can achieve this while maintaining the persistance feature by pre-selecting all the JSON in the editor when opening the app

Start at the app's editor initialization flow and trace how persisted JSON is loaded when the app opens. Check how the editor exposes its current content and selection, then verify that existing JSON remains persisted but is selected on startup so pasted content replaces it and formatting still works with CMD+Enter.

Written by the indexing model from the issue text.