"experimental-features": ["LetUnwrap"], causes a panic

I added `"experimental-features": ["LetUnwrap"],` to my `rescript.json`, which results in:

```
thread 'main' (260424472) panicked at src/watcher.rs:84:14:
Can't initialize build: Could not read rescript.json at /Users/dzakh/code/envio/ui
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 ELIFECYCLE  Command failed with exit code 101.
```