Clarify `enum` and `object literals` usage in `scalajs-dom`

[zetashift]

Enums

Enums currently are defined separately for Scala 2 and Scala 3,not only that these enums, are anonymous objects backed by string fields. Because scalajs-dom wants to stay as close as possible to the JavaScript Web API.

we use enums like this:

PredefinedColorSpace.`display-p3` 

Object literals

These are anonymous objects in Scala 3:

ImageData(200, 100, new { colorSpace = PredefinedColorSpace.`display-p3` })
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the object literal

They are not something like ImageSettings.apply(...) because of sjs-dom goal, stay as close as possible to the JS API.

Not clean, but we should documentate why it is like this, with some example, most likely adding this to CONTRIBUTING.md.