fix(sdk): specify VM.editor and VM.preview types

[fvsch]

Our types for the VM class are lacking:

• any for the editor and preview properties
• Promise<{}> (equivalent to Promise<unknown> in recent TypeScript I believe?) for all methods, including those that actually return a value.
  • In my tests, we return null for all methods on success or when ignored.
  • We return objects of strings or null for getFsSnapshot and getDependencies.

[ggdaltoso]

Nice update! ✨