It thereby reduces code complexity that would arise when setting
many variables using "<$set>".
```
\define helloworld() Hello world!
<$vars greeting="Hi" me={{!!title}} sentence=<<helloworld>>>
<<greeting>>! I am <<me>> and I say: <<sentence>>
</$vars>
```
How this Widget differs from the set widget:
* Variables may be created by using the "key=value" notation
that you already know from widgets like action-setfield.
* You cannot specify a fallback ("emptyValue")
* You cannot use a filter to produce a conditional variable assignement
Original discussion that led to the creation of this widget:
https://github.com/Jermolene/TiddlyWiki5/issues/1610
