Can fragment output location be of `vecN<f16>` type?

I couldn't find a mention of this in the f16 proposal. The PR (https://github.com/gpuweb/gpuweb/pull/2696) also doesn't seem to touch the "validating GPUFragmentState" steps but the problem is that the existing validation talks about vector types without specifying their inner scalar types.

For context, Firefox user bug report: [Bug 2013404](https://bugzilla.mozilla.org/show_bug.cgi?id=2013404)