### Elasticsearch Version

main @ 1266718e1cdef07a8c7cd1a463707825fabf7277

### Installed Plugins

_No response_

### Java Version

openjdk 17.0.8 2023-07-18

### OS Version

Ubuntu 23.;04

### Problem Description

`DISSECT`'s option `append_separator` is not case insensitive in contrast to all other ESQL keywords.

### Steps to Reproduce

This query works:
```
POST _query?format=txt
{
  "query": """
ROW a = "john jacob jingleheimer schmidt"
| DISSECT a "%{+name} %{+name} %{+name} %{+name}" append_separator="---"
  """
}
```
The identical query with upper cased `APPEND_SEPARATOR` does not:
```
POST _query?format=txt
{
  "query": """
ROW a = "john jacob jingleheimer schmidt"
| DISSECT a "%{+name} %{+name} %{+name} %{+name}" APPEND_SEPARATOR="---"
  """
}
```
It fails with `Invalid option for dissect: [APPEND_SEPARATOR]`.

### Logs (if relevant)

_No response_
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/parser/StatementParserTests.java
}
    public void testStatsWithoutAggsOrGroup() throws Exception {
        expectError("from text | stats", "At least one aggregation or grouping expression required in [stats]");
    }
    public void testAggsWithGroupKeyAsAgg() throws Exception {
        var queries = new String[] { """
            row a = 1, b = 2
            | stats a by a
            """, """
            row a = 1, b = 2
            | stats a by a
            | sort a
            """, """
            row a = 1, b = 2
            | stats a = a by a
            """, """
            row a = 1, b = 2
            | stats x = a by a
            """ };
        for (String query : queries) {
            expectError(query, "Cannot specify grouping expression [a] as an aggregate");
        }
    }
    public void testInlineStatsWithGroups() {
        assertEquals(
            new InlineStats(
                EMPTY,
                PROCESSING_CMD_INPUT,
                List.of(attribute("c"), attribute("d.e")),
                List.of(