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
Update changelog in plugin/tohtml.vim comments
vim9script conversion from main Vim repo (#19915 work in progress)