[FEATURE] Support identifiers starting with a digit

Is your feature request related to a problem?

###
POST {{baseUrl}}/test-script-index/_bulk
Content-Type: application/x-ndjson

{ "index": { "_id": 1 } }
{ "value": 10 }
{ "index": { "_id": 2 } }
{ "value": 20 }
{ "index": { "_id": 3 } }
{ "value": 30 }
{ "index": { "_id": 4 } }
{ "value": 40 }

### FAILED
POST {{baseUrl}}/_plugins/_ppl/
Content-Type: application/x-ndjson

{
  "query": "source=test-script-index | stats count() by span(value, 1) as 1v"
}

{
  "error": {
    "reason": "Invalid Query",
    "details": "[1] is not a valid term at this part of the query: '...y span(value, 1) as 1' <-- HERE. Expecting tokens: 'BY'",
    "type": "SyntaxCheckException"
  },
  "status": 400
}

### FAILED
POST {{baseUrl}}/_plugins/_ppl/
Content-Type: application/x-ndjson

{
  "query": "source=test-script-index | eval 2xx=value+1"
}

What solution would you like?
We expected PPL support identifiers starting with a digit
Backtick is workaround

###
POST {{baseUrl}}/_plugins/_ppl/
Content-Type: application/x-ndjson

{
  "query": "source=test-script-index | stats count() by span(value, 1) as `1v`"
}

What alternatives have you considered?
n/a

Do you have any additional context?
n/a

Start by reproducing the two failing PPL requests at /_plugins/_ppl/ and compare them with the backtick workaround. Trace how PPL parses the alias in stats ... as 1v and the assignment in eval 2xx=value+1; the work is done when both forms accept identifiers starting with a digit.