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/components/Toolbar/SetHorzVertDistance.tsx

@@ -14,7 +14,7 @@ import { GetInfoModal, createInfoModal } from '../SetHorVertDistanceModal'
 import { createLiteral, createVariableDeclaration } from '../../lang/modifyAst'
 import { removeDoubleNegatives } from '../AvailableVarsHelpers'
 import { kclManager } from 'lang/KclSinglton'
-import { Selections } from 'useStore'
+import { Selections } from 'lib/selections'
 
 const getModalInfo = createInfoModal(GetInfoModal)