Move `JSPlatform` back to `scalajscrossproject` and deprecate `org.scalajs.sbtplugin.JSPlatform`

[sjrd]

The fact that it is accessible through org.scalajs.sbtplugin.JSPlatform is a remnant of the old design where JSPlatform would be directly provided by sbt-scalajs. The design has now changed, and it will forever be provided by sbt-scalajs-crossproject, so it should be in the package name of the latter.