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;
```

tests/integration/functions/test_function_rowid.rs

@@ -6,6 +6,7 @@ fn test_last_insert_rowid_basic() -> anyhow::Result<()> {
     let _ = env_logger::try_init();
     let tmp_db = TempDatabase::new_with_rusqlite(
         "CREATE TABLE test_rowid (id INTEGER PRIMARY KEY, val TEXT);",
+        false,
     );
     let conn = tmp_db.connect_limbo();
 
@@ -90,7 +91,7 @@ fn test_last_insert_rowid_basic() -> anyhow::Result<()> {
 fn test_integer_primary_key() -> anyhow::Result<()> {
     let _ = env_logger::try_init();
     let tmp_db =
-        TempDatabase::new_with_rusqlite("CREATE TABLE test_rowid (id INTEGER PRIMARY KEY);");
+        TempDatabase::new_with_rusqlite("CREATE TABLE test_rowid (id INTEGER PRIMARY KEY);", false);
     let conn = tmp_db.connect_limbo();
 
     for query in &[