Reimplement ChallengeComboBoxItem

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In phetsims/sun#768, ComboBoxItem is being converted from class to type.

From phetsims/sun#768 (comment):

Hmmm... This makes converting ComboBoxItem to a type a bit more complicated. In ratio-and-proportion:

class ChallengeComboBoxItem extends ComboBoxItem<number> 

... and ChallengeComboBoxItem does some work that a type cannot.

ChallengeComboBoxItem is a bad design. The responsibility for setting the background color of the sim should not be in the item. The item should select a value, and setting the background color based on the value should be handled elsewhere. @zepumph may I change this?

What I said about ChallengeComboBoxItem is true in general. Items should not have responsibilities like this.

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In phetsims/sun#768 (comment), @zepumph said:

Yes that makes sense. Go ahead and make that change. Thanks!

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Since this is a significant change, I published 1.2.0-dev.24 as a baseline before proceeding.

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Done in the above commit.

I noticed that there was a lot of duplication of the ratio constants 1/2, 1/3, 3/4 in ChallengeRatioComboBoxNode. There was an existing Map that mapped ratio to capitalized and lowercase strings. So I extended that Map to include other info about challenges that is needed by ChallengeRatioComboBoxNode. That eliminated the duplication of ratio constants, and facilitates creating the ComboBoxItems by iterating over the map.

@zepumph please review.

[zepumph]

Looks amazing (obviously). Thanks for doing that refactor. I really appreciate. The code is much better.