Companion issue to servo/mozjs#647

• Codegen basis: script: Add cx_no_gc/cx/realm codegen option and demostrate them #40582
• open issue how to do porting and make more demos (and let community take on introducing &mut CurrentRealm everywhere as they should be mostly mechanical changes; by this point we should halt canGC work): draft: https://gist.github.com/sagudev/839382e70631b1eaed5a4d226b830b6e
• use safe JSContext/CurentRealm in codegen (we would still pass it as raw from there): Use safe JSContext/AutoRealm/CurrentRealm in codegen #40716
• start using JSContext/CurrentRealm in WRAP and other codegen related stuff
• pass CurrentRealm as task's argument (this is important source of of get_cx): script: Pass &mut JSContext to tasks #41756
• pass CurrentRealm as callback argument: script: Pass &mut CurrentRealm in callbacks #41536
• port promises to safer apis (realm and JSContext): script: Support new context for promise and add some more helpers to ease migration #41520
• auto add args to functions for promises
• port reflect_dom_object/new to new safe apis
• remove canGC