avoid special characters in word tokens

Transliteration should only consist of ASCII letters
and numbers. Avoid any other characters.

settings/icu_tokenizer.yaml

@@ -21,8 +21,8 @@ transliteration:
     - !include icu-rules/extended-unicode-to-asccii.yaml
     - ":: Ascii ()"
     - ":: NFD ()"
-    - "[^[:Ascii:]] >"
     - ":: lower ()"
+    - "[^a-z0-9[:Space:]] >"
     - ":: NFC ()"
 sanitizers:
     - step: split-name-list