Allow Supplying a filesystem map `fsMap` via `.executeWithFs({ ... })`

This will allow us to supply relative imports, for example you might supply:

```tsx
evalWebWorker.executeWithFs({
  fsMap: {
    "main.tsx": `import manualEdits from "./manual-edits.json"\n//....`,
    "manual-edits.json": "{ ... }"
  },
  entrypoint: "main.tsx"
})
```

/bounty $20