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;
```

sqlite3/src/lib.rs

@@ -126,7 +126,7 @@ pub unsafe extern "C" fn sqlite3_open(
             Err(_) => return SQLITE_CANTOPEN,
         },
     };
-    match limbo_core::Database::open_file(io.clone(), filename, false) {
+    match limbo_core::Database::open_file(io.clone(), filename, false, false) {
         Ok(db) => {
             let conn = db.connect().unwrap();
             *db_out = Box::leak(Box::new(sqlite3::new(io, db, conn)));