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.test.ts

@@ -7,7 +7,8 @@ import {
   ConstraintType,
   getConstraintLevelFromSourceRange,
 } from './sketchcombos'
-import { Selections, ToolTip } from '../../useStore'
+import { ToolTip } from '../../useStore'
+import { Selections } from 'lib/selections'
 import { enginelessExecutor } from '../../lib/testHelpers'
 
 beforeAll(() => initPromise)