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/modifyAst.ts

@@ -1,4 +1,5 @@
-import { Selection, ToolTip } from '../useStore'
+import { ToolTip } from '../useStore'
+import { Selection } from 'lib/selections'
 import {
   Program,
   CallExpression,