A factory for creating keyword marker token filters that prevent tokens from
being modified by stemmers. Two types of keyword marker filters are available:
the SetKeywordMarkerFilter and the PatternKeywordMarkerFilter.
The SetKeywordMarkerFilter uses a set of keywords to denote which tokens
should be excluded from stemming. This filter is created if the settings include
keywords, which contains the list of keywords, or `keywords_path`,
which contains a path to a file in the config directory with the keywords.
The PatternKeywordMarkerFilter uses a regular expression pattern to match
against tokens that should be excluded from stemming. This filter is created if
the settings include keywords_pattern, which contains the regular expression
to match against.