* Add splitLines helper function
I noticed there are a lot of places in our codebase what split strings using a hardcoded `/\r\n|\r|\n/` regular expression. This change extracts that to a new `strings.splitLines` helper
* Update snippetSession.ts
When the core references `vscode`, we only want to import the types and never generate a real import (which will fail to load). Use `import type` to better enforce this
