RangeErrors in link_syntax.dart

[andrechalella]

Markdown source strings that terminate with the following throw RangeError in lib/src/inline_syntaxes/link_syntax.dart:

1. [](<
2. [](<>
3. [](<
4. [](www.example.com "
5. [](www.example.com "\
6. [][\

The problem is lack of EOF checking after several parser.advanceBy(1); lines in that file.

This is related to flutter/flutter#155020.