[Feature] stack based plain-function-helper access to the owner during rendering

[NullVoxPopuli]

I don't want to write an RFC for this right now, as I think it'd be better to prototype the code first

during render, we always know the owner.

so when when invoking a helper (plain function or not), we could do:

// internal psuedo-code
setActiveOwner() // global
invokeHelper(usersHelper) // the users helper
clearActiveOwner(); // global

and then

function usersHelper() {
  let owner = getOwner();

  assert('..', owner);

  let service = owner.lookup('...');

  return service.format(....)
}

this way folks don't need to use class-based helpers or resources to get access to the owner.

[yaxit24]

Please Review #21273.