cmd/cue: better document which filetypes are supported for import/export

[myitcv]

Is your feature request related to a problem? Please describe.

cue help filetypes explains the filetypes that CUE is aware of. However it does not detail which filetypes are supported for import/export:

$ cue import jsonschema: github-workflow.json
$ cue export --out jsonschema github-workflow.cue
unsupported interpretation "jsonschema"

Describe the solution you'd like

Better documentation as part of cue help filetypes

Describe alternatives you've considered

At some point the level of detail required in cue help filetypes might well outgrow the presentation we can afford in a CLI. So perhaps it needs to move to a version-linked URL at some point... ?

On the back of this we should, per https://github.com/cuelang/cue/discussions/897#discussioncomment-794005, create issues for "missing" conversions.

[myitcv]

Here's what such a table might look like:

Tag	Extensions	Description	Import	Export
cue	.cue	CUE source files	n/a	n/a
json	.json	JSON files	Y	Y
yaml	.yaml/.yml	YAML files	Y	Y
jsonl	.jsonl/.ldjson	Line-separated JSON values	?	?
jsonschema		JSON Schema	Y	N
openapi		OpenAPI schema	N	Y
textproto	.textproto	Text-based protocol buffers	?	?
proto	.proto	Protocl Buffer definitions	Y	?
go	.go	Go source files	Via get go	See below
text	.txt	Raw text file	?	Y
binary		Raw binary file	?	?

TODO:

• Add pb in appropriate place
• Provide supporting notes for text and binary caveats
• Provide supporting notes for Go: the options in this space (importing, exporting etc)

[cueckoo]

This issue has been migrated to https://github.com/cue-lang/cue/issues/1012.

For more details about CUE's migration to a new home, please see https://github.com/cue-lang/cue/issues/1078.