feat: retained unclosed open brackets in link reference

[eaglethrost]

	Fix RM-XYZ

🧰 Changes

For a text with an opening bracket right after a complete bracket, the remark library will drop the second opening bracket because it thinks the second part is part of the link reference, will start parsing it as such, but when no closing bracket was found, it only retains the text.

[bar][bar -> [bar]bar

This may look unexpected for users, so this PR updates the link reference transformer to use slicing from the original string to get back the brackets, instead of manually only getting the label

🧬 QA & Testing

• Broken on production.
• Working in this PR app.