Add "isdue" search filter to filter notes with or without a due date set

Problem Statement:

The “due” search filter works great for finding notes due in the future, but when looking for notes due in the past it conflates things without a due date and things with a due date in the past as the same.

For example, if I have 3 tasks, one due last week, one due next week, and one without a due date set, I can search for:

due:day+0

And it returns the one due next week, as expected.

However when I search for

-due:day+0

I don't just get the one with the due date from last week; I also get the one with no due date set, and I can't see any other current syntax that would let me differentiate between those two.

Proposal:

I am proposing to add a new search filter:

“isdue”

It would work much like the current “iscompleted" filter, where “isdue:1” returns all notes with an alarm set, and “isdue:0” returns all notes with no alarm set.

If accepted, I am willing to work on this feature and submit a PR.