Add the character position of a scripting error to error responses.

The contents of the `position` field are experimental and subject to
change.  Currently, `offset` refers to the character location where the
error was encountered, `start` and `end` define a range of characters
that contain the error.

eg.
```
{
  "error": {
    "root_cause": [
      {
        "type": "script_exception",
        "reason": "runtime error",
        "script_stack": [
          "y = x;",
          "     ^---- HERE"
        ],
        "script": "def x = new ArrayList(); Map y = x;",
        "lang": "painless",
        "position": {
          "offset": 33,
          "start": 29,
          "end": 35
        }
      }
```

Refs: elastic#50993

* Check position only for 7.7+

* 7.7 && decrement before assign

* Use correct experimental tag, update doc test responses, off by one yaml

* Do not duplicate error.caused_by in replacement

* Add position under causedby