Multi-file output should honor the output format flag

Currently, evaluation of a Pkl module with multi-file output ignores the output format parameter for the `pkl eval` call (or the `outputFormat` property in case of the Gradle plugin). For example (snippet taken from the [docs](https://pkl-lang.org/main/current/language-reference/index.html#multiple-file-output)):
```pkl
// birds.pkl

pigeon {
name = "Pigeon"
diet = "Seeds"
}
parrot {
name = "Parrot"
diet = "Seeds"
}
output {
files {
["birds/pigeon.out"] {
value = pigeon
}
["birds/parrot.out"] {
value = parrot
}
}
}
```

```
> pkl eval -m output/ -f yaml birds.pkl
output/birds/pigeon.out
output/birds/parrot.out
> cat output/birds/parrot.out
name = "Parrot"
diet = "Seeds"
> cat output/birds/pigeon.out
name = "Pigeon"
diet = "Seeds"
```

This is different from the regular single-file output mode, where `-f` would determine the default renderer in case it is not explicitly specified.

It would probably make sense for the `-f` argument to be honored for multi-file output as well, in the same way as for single-file output - by determining the output format in case an explicit renderer is not defined.

Start with the pkl eval multi-file output path and the Gradle plugin's outputFormat handling; compare them with single-file -f behavior described in the issue and linked language-reference example. Done when -f yaml, and the plugin property, select the default renderer for each generated file unless an explicit renderer is set.

Written by the indexing model from the issue text.