Expose Go APIs for filetypes and encodings

[tvandinther]

Is your feature request related to a problem? Please describe.
I am using CUE to load a CLI config and I would like to easily allow config in various formats by loading it in as CUE. This way I can load and merge part of a configuration in JSON, another in YAML, one part in TOML, and another part in CUE.

Describe the solution you'd like
I want to implement this functionality similarly to how cue export and other commands handle multiple files. I.e. identify the type from extension or by filetype scope toml: data.foo. It would be nice to get this same functionality through an exposed API.

Describe alternatives you've considered
Alternatively I could reimplement all of this logic myself, but this would also then be out of sync with increased filetype compatibility that may come with the upstream cmd/cue.

Additional context
https://cuelang.slack.com/archives/C012UU8B72M/p1771950432593959

[mvdan]

For some context, @myitcv and I have discussed this in the past as well, as we need something similar for the cuelang.org playground. We ended up implementing a tiny version of it:

https://github.com/cue-lang/cuelang.org/blob/632a5e97e0213842c24aaa840227a33381880c0b/playground/impl.go#L158-L178

[mvdan]

It's also worth noting that cue/load is not quite the answer here, because it's much higher level, and it doesn't deal with the encoding side, only the decoding side.