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/python/src/lib.rs

@@ -305,7 +305,7 @@ pub fn connect(path: &str) -> Result<Connection> {
         io: Arc<dyn limbo_core::IO>,
         path: &str,
     ) -> std::result::Result<Arc<limbo_core::Database>, PyErr> {
-        limbo_core::Database::open_file(io, path, false).map_err(|e| {
+        limbo_core::Database::open_file(io, path, false, false).map_err(|e| {
             PyErr::new::<DatabaseError, _>(format!("Failed to open database: {:?}", e))
         })
     }