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/std/sketchcombos.ts

@@ -1,5 +1,6 @@
 import { TransformCallback } from './stdTypes'
-import { Selections, toolTips, ToolTip, Selection } from '../../useStore'
+import { toolTips, ToolTip } from '../../useStore'
+import { Selections, Selection } from 'lib/selections'
 import {
   CallExpression,
   Program,