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: #50993
Stuart Tettemer
James Rodewig
Jack Conradson