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/sketchConstraints.test.ts

@@ -5,7 +5,7 @@ import {
   transformAstSketchLines,
 } from './sketchcombos'
 import { getSketchSegmentFromSourceRange } from './sketchConstraints'
-import { Selection } from '../../useStore'
+import { Selection } from 'lib/selections'
 import { enginelessExecutor } from '../../lib/testHelpers'
 
 beforeAll(() => initPromise)