Update `Thumgeon` button code to use API

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As a 'core' game, Thumgeon should use the Thumby API where possible; in part for best practice/ education.
The custom button handling code should be replaced with API equivalents.
This also avoids the extra import of Pin from machine.

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Corresponding issue for Editor repo is here.

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Additional 'wish list' :

• Test and ensure every dungeon floor has stair exit present during proc'gen'.
• Implement our own PRNG, to avoid Mersenne Twister - less resources and easier to save/ restore state. Perhaps XOR shift?
• Add game auto save, whenever we change the floor. (Or more frequently?).
• Check balance for enemies beyond floor 5.
• Add more utility to 'shirt' and 'pants'?