cmd/cue: qualifiers could include "auto:" to opt back in to filename suffix inferences

[jpluscplusm]

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

In https://cuelang.org/docs/concept/using-the-cue-export-command/inputs/#input-file-encodings we documented that:

There is no way for the [cue export] invocation to re-engage the interpretation of filename suffixes

Meaning that, in invocations such as cue export json: file1 yaml: file2 json: file3.json yaml: file4.yaml, there's no way for the user to say "I need to specify the encoding of file1 and file2, as they're suffix-less, but all the files following them could have their encoding inferred from their filename suffix".

Describe the solution you'd like

In https://cuelang.org/cl/1197464/comment/ed38f30b_df73516e/, @mvdan asked:

Would it make sense to allow cue export auto: foo.yaml as an alias to cue export foo.yaml? We already have a notion of "auto" for build.Interpretation, and there's a default "auto" filetype, even though it appears to not be exposed to the CLI:

auto: interpretations.auto & {
     encoding: *"json" | string
} 

Describe alternatives you've considered

• Rearranging the order of input arguments (not always possible/ideal)
• Manually specifying the qualifiers for all files

Additional context