feat: Support more filetypes in the output_format.

[jaqx0r]

cue help filetypes describes a lot more filetypes we can use for output.

I am trying to generate OpenAPI YAML specs from Cue, per https://cuelang.org/docs/concept/how-cue-works-with-openapi/ and seem to be getting something useful when I add the openapi+yaml output format, supported by this change.

[jaqx0r]

Cue's rules for output format are much more complex than what I'm proposing, so a future version of this change could be a function instead of a dictionary providing the mapping from the full {tag}[+{tag}]... format supported to a file extension.