stringContainsInOrder function
Returns a matcher that matches if the match argument is a string and
contains a given list of substrings in relative order.
For example, stringContainsInOrder(["a", "e", "i", "o", "u"]) will match
"abcdefghijklmnopqrstuvwxyz".
Implementation
Matcher stringContainsInOrder(List<String> substrings) =>
new _StringContainsInOrder(substrings);