String-output

[BobKerns]

In using the plugin to generate non-JSON/YAML files, I found it frustrating to only be able to see the output as a single JSON string.

This PR adds two new commands, and one related feature.

• Jsonnet: Evaluate Expression (String)
• Jsonnet: Evaluate File (String)
• If the first line contains "Output: [name].", is used as the extension for the preview file. This enables syntax highlighting in the target language, if applicable.

I was unable to test the "Evaluate Expression (String)" variant, because I was unable to find an incantation for "Evaluate Expression" that did not give an error, for example, rejecting any simple input with a demand for an identifier. I don't know if this is an LSP bug or my misunderstanding, but Expression works the same as File so I believe my addition to be correct.

[CLAassistant]

All committers have signed the CLA.

[julienduchesne]

I'll test it out! 👁️ 👁️