private String parseToken() {
            int start = position();
            consumeWhile(c -> TOKEN.contains((char) c));

            // Fail if the token was empty.
            if (start == position()) {
                char[] chars = new char[TOKEN.size()];
                int i = 0;
                for (Character character : TOKEN) {
                    chars[i++] = character;
                }
                expect(chars);
            }

            return sliceFrom(start);
        }
Gets the optional structured syntax suffix.

<p>For example given, "application/foo+json", this method returns
"json". Given "application/foo+baz+json", this method returns
"json". Given "application/json", this method returns an empty
{@code Optional}.

@return Returns the optional structured syntax suffix value with no "+".
smithy-utils/src/main/java/software/amazon/smithy/utils/MediaType.java