[BUG] Numbers in scientific notation can not be recognized in PPL

[LantaoJin]

What is the bug?
Scientific notation typically follows the format: $significand × 10^{exponent}$
where the $significand$ is a decimal number, and the $exponent$ is an integer. The significand can have a positive or negative sign, and the exponent can be preceded by 𝑒 or 𝐸, and can also have a positive or negative sign.

Scientific notation can be recognized in SQL, but not in PPL.

How can one reproduce the bug?

POST /_plugins/_sql
{
  "query": """
  SELECT 9e1;
  """
}

returns 90, but

POST /_plugins/_ppl
{
  "query": """
  search source=t | eval `9e1`=9e1 | fields `9e1`
  """
}

throws SyntaxCheckException

Failed to parse query due to offending symbol [e1] at: ... | eval `9e1`=9e1' <--- HERE`

What is the expected behavior?

{
  "schema": [
    {
      "name": "9e1",
      "type": "double"
    }
  ],
  "datarows": [
    [
      90
    ]
  ],
  "total": 1,
  "size": 1
}

What is your host/environment?

• Version 2.16
• Plugins SQL

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Do you have any additional context?
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