Switch to runtime flag for enabling indexes

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;
```

bindings/javascript/src/lib.rs

@@ -71,7 +71,7 @@ impl Database {
         let file = io.open_file(&path, flag, false).map_err(into_napi_error)?;
 
         let db_file = Arc::new(DatabaseFile::new(file));
-        let db = limbo_core::Database::open(io.clone(), &path, db_file, false)
+        let db = limbo_core::Database::open(io.clone(), &path, db_file, false, false)
             .map_err(into_napi_error)?;
         let conn = db.connect().map_err(into_napi_error)?;