generalize normalization step for search query

It is now possible to configure functions for changing the query
input before it is analysed by the tokenizer.

Code is a cleaned-up version of the implementation by @miku.

src/nominatim_api/query_preprocessing/normalize.py

@@ -0,0 +1,26 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2024 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Normalize query test using an ICU transliterator.
+"""
+from typing import cast
+
+from .config import QueryConfig
+from .base import QueryProcessingFunc
+from ..search.query import Phrase
+
+
+def create(config: QueryConfig) -> QueryProcessingFunc:
+    normalizer = config.get('_normalizer')
+
+    if not normalizer:
+        return lambda p: p
+
+    return lambda phrases: list(
+        filter(lambda p: p.text,
+               (Phrase(p.ptype, cast(str, normalizer.transliterate(p.text)))
+                for p in phrases)))