Backwards-incompatible strikethrough changes

[jez]

There are two orthogonal changes here.

1. Recent versions of Vim/Neovim support strikethrough text. We can
   use that instead of underline for ~~foo~~ in Pandoc Markdown
   files.

2. The x̶ is two unicode codepoints:

   • 0x78, which is ASCII x
   • 0x0336, which in UTF-8 is two bytes: 0xCC 0xB6. It "combining
     long stroke overlay" which is a diacritic, modifying the character
     immediately before it.

   Vim does not do a great job when cchar conceals are defined with
   complicated Unicode like this. Even though x̶ renders fine in my
   terminal in Vim, when it's used as a conceal character, only the
   first byte was getting rendered.

   Rather than fix that, and write x directly, I've chosen to delete
   this and treat it like bold or italic regions. Given the
   strikethrough change above, it should be easy to completely
   conceal the strikethrough markers, like is done for bold italic,
   because the strikethrough will disambiguate.

   Users who have already added strikeout in the conceal blacklist
   (see g:pandoc#syntax#conceal#blacklist) will continue to have that
   preference respected. Other users will see a change in the concealed
   preview (see screenshots).

Before (← conceallevel=0 / conceallevel=2 →)

After (← conceallevel=0 / conceallevel=2 →)