feat: support CSS3 break-after:page

[chrisonntag]

Description

The current page break handling on divs only recognized the deprecated page-break-after: always property. However, modern browsers and CSS generators use break-after: page.

This extends the current logic by a more detailed regex to catch the deprecated page-break-after, as well as break-after and handle whitespace variations and !important flags.

Checklist Before Requesting a Review

• I have performed a self-review of my code.
• My code follows the project's coding style and guidelines.
• I have run tests and verified that all existing and new tests pass.
• I have added new tests to cover my changes.

Test issues

I've run all existing and new tests, but both on this branch and the main branch the following test failes:

======================================================================
FAIL: test_local_img (test_h4d.OutputTest.test_local_img)
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Traceback (most recent call last):
  File "/home/christoph/projects/html4docx/tests/test_h4d.py", line 388, in test_local_img
    assert image_found, "No image was found in the document"
           ^^^^^^^^^^^
AssertionError: No image was found in the document

----------------------------------------------------------------------
Ran 84 tests in 6.770s

FAILED (failures=1)

[dfop02]

Approved! Could you just add a Changelog entry before I merge, please?