Fix duplicated LineNr highlight rule
Fix syntax for empty strings in fillchar vars
Fix value extraction from fillchars option
Fix CSS added for styles without class name
Fix "lines" is undefined in Add_diff_fill
Pull more bugfixes from mao-yining's vim9script branch
Fix some exceptions causing test failures:
Bugfixes for "some problems" from mao-yining: github 310b3e209683