Add preliminary support for the anyOf keyword

[LucianBuzzo]

This change introduces support for the anyOf keyword. Other PRs that add support for anyOf/oneOf exist, but are stale and/or incomplete ( #302, #417, #940 ).
The keyword is NOT supported in arrays (using the items keyword).
Options provided in the anyOf keyword are rendered using a new component called AnyOfField, this can be overridden with a custom field using the fields property.

I'd like to follow-up on this PR with work to add support for anyOf in arrays and oneOf. I decided against submitting that work in this PR to help keep reviews and discussion manageable.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
  • If I'm adding a new section, I've updated the Table of Content
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
  • I've run npm run cs-format on my branch to conform my code to prettier coding style
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[LucianBuzzo]

Note that the unit tests are failing due to the New year rollover, which I've fixed/band-aided in a separate PR here #1119

[cyplo]

This looks good to me as a change and would be extremely useful if merged. Thanks ! :)

[LucianBuzzo]

@edi9999 Could you review this PR please?

[LucianBuzzo]

@edi9999 I've addressed your review comments

[glasserc]

I'm really excited to see this! The code mostly looks good and it's got tests, thank you very much!

Documentation would be great :) In particular, I think it's worth calling out that properties provided by anyOf should not overlap any properties from "outside" the anyOf.

[glasserc]

Fantastic, thanks. I'll cut 1.2.0 in a few minutes.

[stathismor]

Is this missing from the playground?

[LucianBuzzo]

@strahius I think the playground hasn't been updated, you can see it in action if you run the playground from source.

[stathismor]

You are right, it just hasn't been deployed yet probably. Thanks a lot, good job!

[epicfaace]

@strahius the playground should be updated now.