fix: use correct enum values for task output mode (#5607)
### Description
Fixes #5598
We weren't correctly (de)serializing the `outputMode` values to our enum
causing prune to fail when parsing an `outputMode` that contained an
`-only` suffix.
### Testing Instructions
Added new unit tests for making sure we can parse `outputMode`s that we
support.
Verify that unit tests match the supported values listed in [our
docs](https://turbo.build/repo/docs/reference/configuration#outputmode)
Co-authored-by: Chris Olszewski <Chris Olszewski>
