support more search operators

[peterbe]

There are more operators (e.g. Unsigned right shift assignment (>>>=)). I picked through the ones that aren't too obscure or ones that could cause confusion. For example, the in operator turns up in too many titles and bodies as a regular stopword so it's probably best to leave it alone.
This'll cover a bunch of more typical ones.

Some examples:

[peterbe]

Note-to-self; Once this lands, remember to run a Dev build because that's what many of us are using for our local kuma docker-compose when we don't want to run a local elasticsearch server.