Provide a simple way to tag YAML value

[YAML tags](https://yaml.org/spec/1.2.2/#24-tags) are a way to attach additional application-specific type information to YAML values. It would be nice to be able to have Pkl produce YAML tags.

This might be done by adding a new class in `pkl:yaml`:
```pkl
class TagDirective {
tag: String(startsWith("!"))
value: Any
}
```

When `YamlRenderer` encounters this type, it would render `tag` as a YAML tag then render `value` as if the `TagDirective` was not present.

Start by reading the pkl:yaml definitions and the YamlRenderer mentioned in the issue, then review the YAML tags specification linked there. The work is done when a TagDirective can carry a tag and value and the renderer emits the tag followed by the value's normal YAML representation.

Written by the indexing model from the issue text.