strange quoting in fn::toString

environment

```
values:
  key:
    fn::toString:
      hello: world
      from: ["pulumi", "esc"]
```
result
```
{
  "key": "\"from\"=\"\\\"pulumi\\\",\\\"esc\\\"\",\"hello\"=\"world\""
}
```
should match [docs](https://www.pulumi.com/docs/esc/environments/syntax/builtin-functions/fn-to-string/)