Addon-knobs: fix knobs function return types

[emilio-martinez]

Issue: The knobs entry functions (text(...), select(...), etc.) are not retaining the proper return value type, i.e., they're falling back to any instead of string, number, or whatever the appropriate type is for any given knob.

What I did

• Use a mixture of conditional types and generics to appropriately route the return value types to the main knob functions.
• Organize the type definitions a bit (It was getting messy. Perhaps a more thorough could take place in a subsequent PR)

How to test

Return values should now come through property for the knob entry functions. Changes should have no runtime impact.

[vercel]

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