refactor selections (#876)

* migrate selection types

* extract selection event into selections.ts

* move code-mirror selection functions into selections.ts

* move more selection logit out of code mirror and engine connection

* add selection functions pure

* tidy up naming

* write a novel about how selections work

* final comments

src/lang/util.ts

@@ -1,4 +1,4 @@
-import { Selections, StoreState } from '../useStore'
+import { Selections } from 'lib/selections'
 import { Program, PathToNode } from './wasm'
 import { getNodeFromPath } from './queryAst'
 import { ArtifactMap } from './std/engineConnection'