Rework exported type names in @frogpond/filter

[drewvolz]

Originally posted by @rye in #6413 (comment)

The type of filters (ListType) doesn't make sense to me when I am reading this. What is that? Are we filtering lists? Can "lists" be passed as filters? I feel like this is an upstream issue with @frogpond/filter, so we can ignore that, but that's why I was confused by this code.

(Also the Type suffix should get dropped from types as a general rule. Types are types, yes. And a more TypeScript-y way would have us make these full-fledged interfaces, I think, but that's a wide-scale change.)

Reply posted by @hawkrives in #6413 (comment)

I want to say that back in Flow, there wasn't as much separation between the "type namespace" and "value namespace", so I tended to suffix things with Type$. This is my fault for bad API design. We should fix it upstream, yeah.