[ngrams] adding function to extract an array of ngrams from a string, with optional special prefixes/suffixes for the edges

src/ngrams.h

@@ -0,0 +1,3 @@
+#include "string_utils.h"
+
+bool add_ngrams(cstring_array *grams, char *str, size_t n, size_t len, bool prefix, bool suffix);