Fix computation of argv_index in best_index

The `filter` methods for extensions affected by this fix expect arguments
to be passed in a specific order. For example, `generate_series` assumes
that if the `start` argument exists, it is always passed to `filter`
first. If `start` does not exist, then `stop` is passed first — but
`stop` must never come before `start`.

Previously, this was not guaranteed: `best_index` relied on constraints
being passed in the order matching `filter`'s expectations.

testing/cli_tests/extensions.py

@@ -320,6 +320,10 @@ def _test_series(limbo: TestTursoShell):
         "SELECT * FROM generate_series WHERE start = 1 AND stop = 10;",
         lambda res: res == "1\n2\n3\n4\n5\n6\n7\n8\n9\n10",
     )
+    limbo.run_test_fn(
+        "SELECT * FROM generate_series WHERE stop = 10 AND start = 1;",
+        lambda res: res == "1\n2\n3\n4\n5\n6\n7\n8\n9\n10",
+    )
     limbo.run_test_fn(
         "SELECT * FROM generate_series(1, 10) WHERE value < 5;",
         lambda res: res == "1\n2\n3\n4",