startWith abstract method
Checks the given character, and if that character might represent
the trailing end of an inline token, that token is returned, otherwise
null is returned.
The given atTextIndex is the index of the character within the
larger text blob.
For example, given a starting character of "", a token might be
returned which is capable of identifying italics "" and bold "**".
But if the character is "#", then null is returned because no
Markdown syntax ends with a "#".
Implementation
UpstreamMarkdownToken? startWith(String character, int atTextIndex);