mechanism for setting Todo keywords/patterns globally

[dhduvall]

When I was making the changes for #13743, it struck me (probably not for the first time) that this was sort of tedious. Every syntax file which has xxxTodo defined has a different set of words, some have patterns, etc.

I was wondering if there would be an appetite for a way to make that list global, settable by the user.

It seemed like the most straightforward thing would be to have a single syntax definition (Todo, I guess) that would replace all the xxxTodo items, and have that set to a reasonable default somewhere. Then a user would simply have to have

autocmd Syntax * syn keyword Todo contained HELP!!!

However, it wasn't clear to me how the Syntax autocommand was supposed to work—since all of them get cleared in synload.vim, none which are set in .vimrc survive to the point where they'd actually be used. I ended up sticking it in mysyntaxfile, because that's read after synload.vim, but since that file is deprecated, it doesn't seem like the right place.

Any thoughts?