fix: Prevent crash from malformed links

[lebojo]

This pull request improves the robustness of the MarkdownLink element parsing in MarkdownLink.swift by adding checks to prevent crashes on malformed links and by refactoring code to use named variables for better readability and maintainability.

Concretely:

• Added guard statements to prevent crashes when parsing malformed markdown links by checking the number of ranges and valid locations before proceeding.

[demianturner]

Thanks @lebojo — this fix works well for me and resolves the crash. I’ve applied it in my fork.