Is your feature request related to a problem? Please describe.
Sometimes 3rd party systems require annotating configurations via comments.
For example, cloud-init requires that its yaml files have a she-bang line like this:

#cloud-config
<some yaml stuff>
......

See here https://cloudinit.readthedocs.io/en/latest/topics/examples.html for more examples.

Describe the solution you'd like
File test.cue

//cloud-config
foo: "bar"

hello: {
        // occupy
        world: "from mars?"
}

when exported via command cue export --out yaml --preserve-comments test.cue should produce something like this:

# cloud-config
foo: bar
hello:
  # occupy
  world: from mars?

Describe alternatives you've considered
There is an easy workaround when wrapping cue-cmd via bash script. It's suboptimal but it's not a blocker.

Additional context
Bonus point: Some comments may be relevant to cue language itself thus we do not really want to export them.
It could be nice to have a convention to export whitelisted comments. For example, only comments starting from /// will be exported.