test/cases/0160-string: add tests and ldoc description for split_by_char
test/cases/0160-string: add tests and small fix for split_by_offset
test/cases/0160-string: add tests and small fix for count_substrings
test/cases/0160-string: add tests and ldoc description for kv_concat
test/cases/0160-string: add tests for escape_for_json
test/cases/0160-string: add tests and ldoc description for
get_escaped_chars_in_ranges