Improved heading detection by adding isolated bold case

[hnc-leebd]

For the dataloader, I added a case that detects titles with the same font name, a large font size, and bold style, which are occasionally seen in papers.

Since bold style already receives bonus points for properness, I added logic to determine if a short, single-line text node is isolated and assign bonus points.

This logic requires determining the bounding box, which involves expanding the overall test code.

• Improved heading detection by adding isolated bold case.
• Update NodeUtilsTest to now also consider the bounding box of each text node.
• Add AssertJ as a test-scoped dependency so fluent assertions are available.

[hnc-leebd]

I will push the revised commit.

[hnc-leebd]

@MaximPlusov
I squashed the third commit and the new commit.
I've incorporated all of the feedback you gave me.
Thank you so much for the detailed review!