Bug Description

When using GenericAttributesParser, if an attribute block (e.g., {.center}) appears on a line by itself immediately before a block element (such as a paragraph), the resulting block element includes a leading newline (\n) in its content.

Expected Behavior

According to the Pandoc Markdown specification:

Attributes that occur immediately before a block element, on a line by themselves, affect that element.

The attribute block should be associated with the following block element, but the content of the block element should not include an extra leading newline.

How to Reproduce

Steps:

1. Create a markdown file containing:

   {.center}
   A paragraph
   

2. Parse this with Markdig (with GenericAttributes enabled)
3. Observe that the paragraph block contains a leading \n in its content.

Actual Behavior

The paragraph block has a leading newline before the text, which is preserved in the rendered HTML or output.

Expected Output

The paragraph should have the attribute applied, but its content should be:

A paragraph

without any leading newline.

Reference

Pandoc Markdown spec: https://pandoc.org/MANUAL.html#extension-attributes

Additional Context

This may be due to how the parser transitions from the attribute block to the following block, and does not trim the whitespace. This leads to non-compliance with Markdown attribute extension behavior as described in Pandoc.

Please consider aligning this behavior with the Pandoc spec by trimming/removing the leading newline when attaching attributes from an attribute-only paragraph to the next block.