Complete Playground Page Manifest implementation

[pietbrauer]

Playground pages can be modified with various arguments (Source]:

Argument	Type	Description	Required
Hints	Array of dictionaries	An array of static hints and optional spoilers. See Hints Key.
LiveViewEdgeToEdge	Boolean	Sets the live view to extend past the margin in the area used for live views. See LiveViewEdgeToEdge Key.	✓
LiveViewMode	String	Sets the live view to show even when it is not running. See LiveViewMode Key.	✓
Name	String	The name displayed in the user interface. See Name Key.	✓
PlaygroundLoggingMode	String	Controls the display of results in playground pages. See PlaygroundLoggingMode Key.
PosterReference	String	An image shown before the live view runs. See PosterReference Key.

Currently we support LiveViewEdgeToEdge, LiveViewMode and Name.

The manifest.plist is written in lib/playgroundbook/page_writer.rb and can easily be extended with the missing parameters described above.

This can be a good first step for someone who is looking to contribute.