Add custom formatting for Dynamic Component

[codemayank]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Feature Update.

• What is the current behavior? (You can also link to an open issue here)
  Addresses Custom formatting for Dynamic #490

• What is the new behavior (if this is a feature change)?
  Dynamic Component will accept a prop map that will map a give value to the value prop of the component. e.g.

[Dynamic value:Var1 min:50 max:100 step:10 map:`['Red', 'Red', 'Blue', 'Green']`/]
[Dynamic value:Var2 min:0 max:4 step:1 map:`(Var2 % 2 === 0) ? 'even' : 'odd' ` /]

This will be the output of above

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No.

• Other information:

[mathisonian]

Thanks @codemayank! I've left a couple comments that I think will help make the API match the use case that @suhr and @rowanc1 were describing in #490

[mathisonian]

After thinking about it a bit more, passing in a reactive expression rather than a function seems more idyllic.

I pushed an update to make this work, and made a short test post implementing some of @rowanc1's examples. See here:

https://idyll.pub/post/dynamic-test-3f0ebd38f8d30970b1da2f89/

Since its not a function anymore and there isn't an easy way to differentiate it from our existing format option, I've changed it back to accept a second prop, display. See the example above to see this in action, I think this seems like a pretty nice API.

[rowanc1]

Thanks for implementing this @mathisonian and @codemayank!

I started a similar project before I discovered Idyll, I wrote up a bit of a review of the differences in my thinking/features so far:

curvenote/article#8

I would be very curious on your thoughts on this. Really awesome work on this project/language.