Shortcuts for importing and parsing YAML (and maybe JSON and other stuff)

[sbarzowski]

Tanka used to have a feature which automagically parsed YAML when importing files with yaml extension. This turned out to cause problems with importstr and was disabled.

We can think about providing some kind of shortcut upstream. For example importyaml 'foo.yaml'.

Note that it's always possible to import as string and parse explicitly – std.parseJson(importstr 'foo.json').

See also: https://github.com/google/go-jsonnet/pull/420