Consider enabling bigram_index to improve autocomplete

@donhardman is already working on this.

Since Sep 26, 2024.

Here

The autocomplete results could be better if they included `native support`. It's currently impossible likely because `bigram_index` is not enabled. If we do it, the results may be better. E.g.:

```
mysql> drop table if exists t; create table t(f text) min_infix_len='2' bigram_index='all'; insert into t(f) values('abc def'),('abc'),('def'); call autocomplete('ab', 't');
--------------
drop table if exists t
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
create table t(f text) min_infix_len='2' bigram_index='all'
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
insert into t(f) values('abc def'),('abc'),('def')
--------------

Query OK, 3 rows affected (0.01 sec)

--------------
call autocomplete('ab', 't')
--------------

+---------+
| query |
+---------+
| abc |
| abc def |
+---------+
2 rows in set (0.01 sec)
```

Let's experiment with it and consider enabling it for the demo.