scowl: support installing just words.txt

The expression now supports having `words.txt` in some place without tens
and tens of megabytes of all the wordlist and spelling dictionaries. Set
`singleWordlist` parameter to the string of region and size settings. For
example:
```
scowl.override{singleWordlist = "en-gb-ise 60";}
```

Should be useful for #34486