Makes it easier to test the feature:
```
$ cargo run -- --experimental-indexes
Limbo v0.0.22
Enter ".help" for usage hints.
Connected to a transient in-memory database.
Use ".open FILENAME" to reopen on a persistent database
limbo> CREATE TABLE t(x);
limbo> CREATE INDEX t_idx ON t(x);
limbo> DROP INDEX t_idx;
```
- `Update` query doesn't update `n_changes`. Let's make it work
- Add `InsertFlags` to add meta information related to insert operations
- For update query, add `UPDATE` flag
- Currently, the update query executes `Insn::Delete` and `Insn::Insert`
internally, it increases `n_change` by 2. So, for the update query,
let's skip increasing `n_change` for the `Insn::Insert`
https://github.com/tursodatabase/limbo/issues/1681
Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>
Closes#1683
