* Add action-setmultiplefields and setmultiplevariables, and extend action-sendmessage
* Add getfield operator
* Remove getfield operator
See discussion at https://github.com/Jermolene/TiddlyWiki5/pull/6130#issuecomment-949911439
* Add docs
* Adjust whitespace
* Add support for assigning multiple indexes to action-setmultiplefields
Using `$name` and `$value` attributes allows more flexibility in how
parameter names are specified, allowing parameter names that are not
valid attribute names.
In 5.1.3 we introduced the idea that the `event.param` variable could
optionally be a hashmap, allowing multiple values to be passed with the
message.
This change moves the hashmap to a separate `event.paramObject`
variable, allowing us to pass both a hashmap and a param string.
