Add support for Pandoc raw_attribute syntax

In addition to the ```lang syntax for beginning a fenced code block with
syntax highlighting, support the ```{=lang} syntax used by Pandoc to
include raw code of that language (e.g. raw HTML code if the Markdown
file will be converted to HTML).

https://pandoc.org/MANUAL.html#extension-raw_attribute

Author: psvenk

syntax/markdown.vim

@@ -119,8 +119,8 @@ if main_syntax ==# 'markdown'
     if has_key(s:done_include, matchstr(s:type,'[^.]*'))
       continue
     endif
-    exe 'syn region markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(`\{3,\}\)*\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\.','','g') . s:concealends
-    exe 'syn region markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(\~\{3,\}\)*\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\.','','g') . s:concealends
+	exe 'syn region markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(`\{3,\}\)*\s*\%({.\{-}\.\)\={\==\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\.','','g') . s:concealends
+    exe 'syn region markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(\~\{3,\}\)*\s*\%({.\{-}\.\)\={\==\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\.','','g') . s:concealends
     let s:done_include[matchstr(s:type,'[^.]*')] = 1
   endfor
   unlet! s:type