Eagerly create jvm target for KMP

[Goooler]

Please don't call jvm(). Because this method declares a jvm target in KMP project. And users should do that explicitly.
More over, they can specify a custom name for their jvm target.
The correct API is to lazily iterate over targets collection. And check for KotlinJvmTarget instance.

kmpExtension.targets.configureEach { target ->
   if (target is KotlinJvmTarget) // do configuration here
}

Originally posted by @antohaby in #1280 (comment)