Allow custom definitions of in-word characters

[AlbertWeichselbraun]

This pull request allows specifying custom in-word characters for words by extending isPartialMatch in Trie.java, which improves the libraries flexibility in defining word boundaries.

Example:

// create a searcher that allows numbers and hyphens in words.
StringSearcher searcher = StringSearcher.builder().addSearchString("ER-Models")
                .addSearchString("Database").addSearchString("C2")
                .setIsInWordCharacter(ch -> Character.isAlphabetic(ch) || Character.isDigit(ch) || ch == '-').build();

Both the currently used onlyWholeWordsWhiteSpaceSeparated and onlyWholeWords flags can be expressed with a corresponding inWordCharacterexpression.
=> StringSearcherConfig.java has been adapted to continue supporting both flags as well.