[ES|QL] Improve STATS command summary extraction (elastic#199796)

## Summary

Partially addresses elastic#191812

- Correctly extracts summary from of fields from the `BY` clause of
`STATS` command.
- The `.summarize()` command now returns `newFields` and `usedFields`
properties. The `newFields` is a list of newly created fields by the
`STATS` command. The `usedFields` is a list of all fields which were
used by the `STATS` command.
- Improves parameter node handling.

### Example

Extract all "new" and "used" fields from all `STATS` commands:

```ts
const query = EsqlQuery.fromSrc('FROM index | STATS a = max(b), agg(c) BY d');
const summary = mutate.commands.stats.summarize(query);

console.log(summary.newFields);     // [ 'a', '`agg(c)`' ]
console.log(summary.usedFields);    // [ 'b', 'c', 'd' ]
```

### Checklist

Delete any items that are not applicable to this PR.

- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

### For maintainers

- [x] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#_add_your_labels)

(cherry picked from commit d276b48)

# Conflicts:
#	packages/kbn-esql-ast/src/mutate/commands/stats/index.test.ts
#	packages/kbn-esql-ast/src/mutate/commands/stats/index.ts

packages/kbn-esql-ast/src/ast/helpers.ts

@@ -14,6 +14,7 @@ import type {
   ESQLFunction,
   ESQLIntegerLiteral,
   ESQLLiteral,
+  ESQLParamLiteral,
   ESQLProperNode,
 } from '../types';
 import { BinaryExpressionGroup } from './constants';
@@ -48,6 +49,9 @@ export const isIntegerLiteral = (node: unknown): node is ESQLIntegerLiteral =>
 export const isDoubleLiteral = (node: unknown): node is ESQLIntegerLiteral =>
   isLiteral(node) && node.literalType === 'double';
 
+export const isParamLiteral = (node: unknown): node is ESQLParamLiteral =>
+  isLiteral(node) && node.literalType === 'param';
+
 export const isColumn = (node: unknown): node is ESQLColumn =>
   isProperNode(node) && node.type === 'column';
 

packages/kbn-esql-ast/src/mutate/README.md

@@ -60,3 +60,21 @@ console.log(src); // FROM index METADATA _lang, _id
     - `.remove()` — Remove a `LIMIT` command by index.
     - `.set()` — Set the limit value of a specific `LIMIT` command.
     - `.upsert()` — Insert a `LIMIT` command, or update the limit value if it already exists.
+  - `.stats`
+    - `.list()` — List all `STATS` commands.
+    - `.byIndex()` — Find a `STATS` command by index.
+    - `.summarize()` — Summarize all `STATS` commands.
+    - `.summarizeCommand()` — Summarize a specific `STATS` command.
+
+
+## Examples
+
+Extract all "new" and "used" fields from all `STATS` commands:
+
+```ts
+const query = EsqlQuery.fromSrc('FROM index | STATS a = max(b), agg(c) BY d');
+const summary = mutate.commands.stats.summarize(query);
+
+console.log(summary.newFields);     // [ 'a', '`agg(c)`' ]
+console.log(summary.usedFields);    // [ 'b', 'c', 'd' ]
+```