luci-base: luci.js: properly handle nested Class.super() calls

Introduce a unique __id__ attribute per class and use that, together with
the symbol name being looked up to, to form a unique key which is used to
store the prototype scope pointer.

This fixes cases where a call to super() invokes a procedure which is
calling super() as well on a different class or symbol.

Fixes: #3316
Signed-off-by: Jo-Philipp Wich <jo@mein.io>