Several additions and improvements

[MenBrial]

Several additions/improvements:

• Test for InsertToc command.
• Fix syntax refresh for hovering windows (also on WinLeave).
• Fix single '<' syntax highlighting.
• Change new_list_item_indent from 4 to sw.
• Simplify syntax folding.
• Add Atx header concealment.
• Change italic and bold syntax expressions to add end-concealment.
• Add html comment concealment.
• Add fenced code block delimiter concealment.
• Add character concealment utility.
• Add option for controlling link concealment.