Backwards-incompatible strikethrough changes

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 disabled. Other users will see a change in the concealled
    preview (see screenshots).

Author: jez

syntax/pandoc.vim

@@ -41,7 +41,6 @@ if &encoding ==# 'utf-8'
                 \'image': '▨',
                 \'super': 'ⁿ',
                 \'sub': 'ₙ',
-                \'strike': 'x̶',
                 \'atx': '§',
                 \'codelang': 'λ',
                 \'codeend': '—',
@@ -60,7 +59,6 @@ else
                 \'image': 'i',
                 \'super': '^',
                 \'sub': '_',
-                \'strike': '~',
                 \'atx': '#',
                 \'codelang': 'l',
                 \'codeend': '-',
@@ -393,8 +391,7 @@ call s:WithConceal('superscript', 'syn match pandocSuperscriptMark /\^/ containe
 " }}}3
 
 " Strikeout: {{{3
-syn region pandocStrikeout start=/\~\~/ end=/\~\~/ contains=@Spell,pandocAmpersandEscape keepend
-call s:WithConceal('strikeout', 'syn match pandocStrikeoutMark /\~\~/ contained containedin=pandocStrikeout', 'conceal cchar='.s:cchars['strike'])
+call s:WithConceal('strikeout', 'syn region pandocStrikeout matchgroup=pandocOperator start=/\~\~/ end=/\~\~/ contains=@Spell,pandocAmpersandEscape keepend', 'concealends')
 " }}}3
 
 " }}}2
@@ -699,11 +696,10 @@ function! s:SetupPandocHighlights()
   hi def link pandocNoFormattedAttrs Comment
   hi def link pandocSubscriptMark Operator
   hi def link pandocSuperscriptMark Operator
-  hi def link pandocStrikeoutMark Operator
   if g:pandoc#syntax#style#underline_special == 1
       hi pandocSubscript gui=underline cterm=underline
       hi pandocSuperscript gui=underline cterm=underline
-      hi pandocStrikeout gui=underline cterm=underline
+      hi pandocStrikeout gui=strikethrough cterm=strikethrough
   endif
   hi def link pandocNewLine Error
   hi def link pandocHRule Delimiter