JSON key "<<" is converted to "!!merge <<" in YAML

Hello.

Unexpected output:

```
$ echo '{"<<": 3}' | yj -jy
!!merge <<: 3
```

This doesn't round-trip, which feels wrong to me:

```
$ echo '{"<<": 3}' | yj -jy | yj -yj
Error parsing YAML: map merge requires map or sequence of maps as the value
```

Expected output:

```
$ echo '{"<<": 3}' | yj -jy
"<<": 3
```