* Adding case where PrivateName is used inside computed property.
* Adding proper description to early errors on usage of undeclared private names inside computed property.
The template file violated the conventions from "src/class-elements/conventions.md".
Specifically there must be a global `C` variable, but default exports don't
create a global variable binding. There is no easy way to refer to the current
module and by that retrieve the default export, therefore the template was
removed.
