Aggregation editor -> new stage -> Default stage is $match.
When you click to the the select list field, you have to always start with the $ character and write the starting letter of the operator name, to "complete it" with down arrow press.

All the stage operators begin with $ character so it will be nice if you'll be able to autocomplete just with the beginning letters, and add the $ character on the background.

Example:

writing gro
press down arrow
in the input appears $group.

I know this is the cherry on the pie, but i see it as relatively easy to implement, and if you write too much multistage aggregations a day, it saves some "shift-presses" :-)