fix: export exists and prox_str from top-level paradedb package

Both `exists` and `prox_str` were implemented in
`paradedb/sqlalchemy/search.py` but were not re-exported in
`paradedb/__init__.py`, causing `ImportError` when users did
`from paradedb import exists` or `from paradedb import prox_str`.

Added both symbols to the import block and to `__all__`, in alphabetical
order consistent with the surrounding exports.

Author: philippemnoel

paradedb/__init__.py

@@ -13,6 +13,7 @@
 from .sqlalchemy.search import (
     ProximityExpr,
     all,
+    exists,
     match_all,
     match_any,
     more_like_this,
@@ -21,6 +22,7 @@
     phrase_prefix,
     prox_array,
     prox_regex,
+    prox_str,
     proximity,
     range_term,
     regex,
@@ -37,6 +39,7 @@
     "all",
     "assert_indexed",
     "describe",
+    "exists",
     "match_all",
     "match_any",
     "more_like_this",
@@ -49,6 +52,7 @@
     "phrase_prefix",
     "prox_array",
     "prox_regex",
+    "prox_str",
     "proximity",
     "range_term",
     "regex",