## Summary

Up till now, we had to define our own lexer rules for our client-side
ES|QL validation. This was because we were using an unofficial ANTLR
package (before the official ANTLR had typescript support).

Now that we are using the official ANTLR library (as of
#177211), we no longer have to
encode case insensitivity into the lexer rules themselves because the
[`caseInsensitive` option](antlr/antlr4#3399) is
now available to us.

This means we can adopt the very [same
definitions](https://github.com/elastic/elasticsearch/blob/343b1ae1ba74fbf2e75c29adddb2790312dd680b/x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.g4)
that Elasticsearch uses as long as we set `caseInsensitive`
(Elasticsearch handles case insensitivity at runtime).

### Checklist

- [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