[BUG][PPL] `isnotnull()` works but `ISNOTNULL()` doesn't work

[joshuali925]

What is the bug? isnotnull() works but ISNOTNULL() doesn't work

How can one reproduce the bug? Steps to reproduce the behavior:

1. Go to playground
2. Run

POST _plugins/_ppl
{
  "query": "source=.kibana | where ISNOTNULL(type)"
}

3. see "{\n \"error\": {\n \"reason\": \"Invalid Query\",\n \"details\": \"unsupported function name: isnotnull\",\n \"type\": \"ExpressionEvaluationException\"\n },\n \"status\": 400\n}"
4. change ISNOTNULL to lower case, query runs

What is the expected behavior? A clear and concise description of what you expected to happen.

What is your host/environment?

• OS: [e.g. iOS] linux
• Version [e.g. 22] 2.11.0
• Plugins

Do you have any screenshots? If applicable, add screenshots to help explain your problem.

Do you have any additional context? Add any other context about the problem.

[LantaoJin]

Thanks for this issue, I am going to fix this.