Function proposal - parseJson()

Given a string containing embedded json, parse the json, and return the result.

This might be implemented something like:

```js
registerFunction('parseJson', ([json]) => JSON.parse(json), [{types: [TYPE_STRING]}]);
```

Examples:

```js
jmespath.search("\"true\"", "parseJson(@)")
// OUTPUTS: true

jmespath.search('"{\"a\":1, \"b\":\"c\"}"', "parseJson(@)")
// OUTPUTS {"a":1, "b":"c"}
```

The pattern of embedding json inside of strings in json is, unfortunately, somewhat common. For example, in AWS APIs.