Plan the cleanup of Js.Foo2 -> Js.Foo as the default

[cristianoc]

There are a few Js.Foo2 modules that are t-first.
Complete these modules, and move them to be the default.
Give an escape hatch for backwards compatibility even though this will necessarily be a breaking change.

E.g. https://github.com/rescript-lang/rescript-compiler/pull/5709/files#r985058585

[cristianoc]

This has moved to the core library.