Link to astexplorer in the spec

[sbarzowski]

@Alxandr implemented AST Explorer plugin for Jsonnet. It may be pretty useful for tooling development.

More details in #804.

[Alxandr]

Do note that this is a parser I made, and has not been vetted. There are definitely some rough edges (especially around errors in the input), and it currently does not deal well with slices a[from:to:step].

On a side-note, I'm also planning to look at an LSP implementation.

[Alxandr]

FYI: fkling/astexplorer#505

[Alxandr]

I also added a transformer to the core language to astexplorer. It can be seen here. Pick jsonnet as language, and pick core language in the transformer list. The core language visualization is custom, as is the core language itself. It's fairly true to the core language described in the spec, with the exceptions that I

1. bind variables (so unique variables have unique identities).
2. keep import statements.
3. keep the +: (and similar) operators for object fields.

I'm not sure I've done all the desugaring correctly, so I made this to make it easier to see what the output of the desugaring was. If you feel like, feel free to test it out and report discrepancies.