[Initial Feedback] Recursive Tree Traversal

A very useful feature of XSLT is to supports recursive tree traversal using XPath.
I believe such a feature would be a great addition to JMESPath.

I have drafted [a very ugly working prototype](https://github.com/jmespath-community/typescript-jmespath/tree/descendant) for what could be a new `descendant-expression`

# Specification

A new `descendant-expression` will be added as an alternative to the `expression` grammar rule:

```abnf
descendant = ".." identifier
descendant-expression = ( expression ) 1*( descendant )

expression =/ descendant-expression
```

A `descendant-expression` is a projection that returns an array with all the elements of the JSON input with the specified name.

## Examples

Given the following JSON input:

```json
{
"name": "first",
"top": { "name": "second" },
"nested": {
"name": [
1, "string", false,
{ "name": "third" }
]
}
}
```

The following expressions are possible:

|Expression|Result
|---|---
| `` ..name `` | `` ["first","second",[1,"string",false,{"name":"third"}],"third"] ``
| `` ..name.length(@) `` | `` [5, 6, 4, 5] ``
| `` ..name..name `` | `` ["third"] ``

## Compliance Tests

This JEP standardizes a syntax that was formely not valid. For this reason, one of the syntax.json compliance tests must be changed or removed altogether:

```patch
{
"expression": ".foo",
"error": "syntax"
},
{
"expression": "foo..bar",
- "error": "syntax"
+ "result": null
},
```

A new file `descendant.json` will be added to the compliance test suite.