Dim: Add a search for stackable items

[joshminnie]

Similar to the behavior of level and light add a search flag for stacked items.

Examples

stack:<3
stack:>3
stack:>=3
stack:<=3

[kyleshay]

currently we have is:stackable, what would be the use in knowing if you have >/=/< a certain amount?

[duckmanBR]

@kyleshay because in D2 you can exchange 3 equal blue mods for a purple one. So people are moving all blue mods to vault and then moving back only the stacks with 3 (or more)...

[kyleshay]

Linking #2255

[duckmanBR]

I implemented that but I messed my fork so it could take a little longer...