[FEATURE] Add possibility to make builder non-public

[annam002]

The @builder annotation always creates a public builder class with public methods.
This is not always wanted; for package-private and protected classes, it does not make sense.
It would be nice to be able to define the access level for the builder, just like you can do it with constructors.

@Builder(access = AccessLevel.PACKAGE)
class TestMe {
	private final String test;
}

would generate

@Builder(access = AccessLevel.PACKAGE)
class TestMe {
	private final String test;

        static final class TestMeBuilder {
            //builder methods here
	}

}