Support oneOf and anyOf as alternative of enum/enumNames

[fbessou]

Reasons for making this change

As mentioned by @dlax in #532 enumNames keyword is not in the JSON Schema specification (neither draft-04 nor draft-06).
Another way to support choices schemas is to write them with the following structure :

{
  oneOf: [{
    enum: ["foo"],
    title: "Foo"
  }, {
    enum: ["bar"],
    title: "Bar"
  }]
}

or with draft-06's new const keyword:

{
  oneOf: [{
    const: "foo",
    title: "Foo"
  }, {
    const: "bar",
    title: "Bar"
  }]
}

This PR adds basic support for oneOf/anyOf as an alternative to the non standard enumNames keywords.

Checklist

• 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

[glasserc]

This is fantastic work, thanks! My only real complaints:

• isConstant could use some JSDoc saying that it's used on schemas (maybe something like "Checks to see if a schema only matches a single constant value")

• isSelect probably needs a better name and/or JSDoc. (Maybe the name is OK since isMultiSelect exists, although I'm not crazy about that name either.)

Sorry it took so long to look at this!

[glasserc]

referenced

[glasserc]

How about isSelect(schema.items, definitions) instead?

[fbessou]

Currently there isn't any JSDoc comment in the react-jsonschema-form sources. Should I add a simple comment or a JSDoc comment ?

[glasserc]

We don't have any code that does anything specific with JSDoc comments and actually most functions are not documented using JSDoc, but adding a JSDoc comment is fine. It doesn't have to be complicated -- the example on validateFormData is fine.

[fbessou]

@glasserc, I made all the requested changes except the renaming of isSelect for which I haven't found a better name.

[glasserc]

Thanks!

[glasserc]

Released in v0.50.0.