Discuss adding a new known format/ emitter option type for `path`/`relativePath`

From #2241

This https://github.com/microsoft/typespec/issues/2493 issue will add `absolute-path` as a new format for the options. But remains the question if we need a `relative-path` or generic `path`

```tsp
dir: Typespec.EmitterOptions.relativePath<"emitter-output-dir">
```
Arguments for:
- It would be some official documentation that an option is going to be resolved relative to what
- Would resolve the relative path automatically for the emitter

There is also the meaning of relative:
- from the cli, one would think it is relative to the `cwd`
- from the config, one would think it is relative to the config file itself

With those, do we even want to allow relative path at all. It is quite confusing otherwise.