Create .native.js and .js so j2cl supports `org.gwtproject.safecss.ForceCheckValidStyles`

[niloc132]

This will allow projects to enable runtime checks even in production code, just through a compiler flag (instead of invoking org.gwtproject.safecss.shared.SafeStylesHostedModeUtils#setForceCheckValidStyle)

Probably we should also change this name, so that we don't have to define the whole namespace, can worry about that later.