[jnigen] ☂️ General refactorings #1581

Before 1.0 I would like to do some general quality of life clean ups, this is the umbrella issue of it.

• Format the test descriptions correctly
• Remove abstract class Jni with basically only static fields and members, Jni.spawn can simply be spawnJvm
• Remove ProtectedJniExtensions, have all members as global internal methods
• Use consistent cases for the C API
• Improve the folder structure of jnigen
• Dramatically reduce the amount of exported symbols from package:jni
• Rename $MethodInvocation, instead use import prefixes to prevent name collision
• Now that the implementation class for an interface called Foo is called $Foo instead of $FooImpl they can theoretically collide with another interface called FooType, where the type class of Foo is $FooType, and the implementation class of FooType is also $FooType. If we make the type class to be Foo$Type instead, it can collide with an inner class of Foo named Type. So maybe$Foo$Type?
• Possibly make type classes @internal, users should instead use Foo.type.
• Change T extends JObject's late final $type to be final $type. It's not a big deal to possibly create an extra type object as the level of generics are not that deep so the type is usually just a const anyway.
• From a naming collision perspective, there is no reason to prepend every parameter and local variable with $s. It's not necessary to read the generated code, but we also shouldn't go out of our way to make both the generator and the generated code uglier.
• Make all multiline strings use '''
• Separate the API and implementation of JNIgen generated files to make the bindings look cleaner

More to be added