patch 9.1.0276: No pandoc syntax support

Problem:  No pandoc syntax support
Solution: Add pandoc syntax and compiler plugins
          (Wu, Zhenyu, Konfekt)

closes: #14389

Co-authored-by: Konfekt <Konfekt@users.noreply.github.com>
Signed-off-by: Wu, Zhenyu <wuzhenyu@ustc.edu>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/compiler/pandoc.vim

@@ -0,0 +1,58 @@
+" Vim compiler file
+" Compiler:     Pandoc
+" Maintainer:   Konfekt
+"
+" Expects output file extension, say `:make html` or `:make pdf`.
+" Passes additional arguments to pandoc, say `:make html --self-contained`.
+
+if exists("current_compiler")
+  finish
+endif
+
+if exists(":CompilerSet") != 2		" older Vim always used :setlocal
+  command -nargs=* CompilerSet setlocal <args>
+endif
+
+let s:keepcpo = &cpo
+set cpo&vim
+
+let current_compiler = 'pandoc'
+
+" As of 2024-04-08 pandoc supports the following text input formats with
+" an ftplugin on Github:
+let s:supported_filetypes =
+      \ [ 'bibtex', 'markdown', 'creole', 'json', 'csv', 'tsv', 'docbook',
+      \   'xml', 'fb2', 'html', 'jira', 'tex', 'mediawiki', 'nroff', 'org',
+      \   'rtf', 'rst', 't2t', 'textile', 'twiki', 'typst', 'vimwiki' ]
+" .. and out of those the following are included in Vim's runtime:
+" 'xml', 'tex', 'html', 'rst', 'json', 'nroff', 'markdown'
+
+silent! function s:PandocFiletype(filetype) abort
+  let ft = a:filetype
+  if ft ==# 'pandoc'
+    return 'markdown'
+  elseif ft ==# 'tex'
+    return 'latex'
+  elseif ft ==# 'xml'
+    " Pandoc does not support XML as a generic input format, but it does support
+    " EndNote XML and Jats XML out of which the latter seems more universal.
+    return 'jats'
+  elseif ft ==# 'text' || empty(ft)
+    return 'markdown'
+  elseif index(s:supported_filetypes, &ft) >= 0
+    return ft
+  else
+    echomsg 'Unsupported filetype: ' . a:filetype ', falling back to Markdown as input format!'
+    return 'markdown'
+  endif
+endfunction
+execute 'CompilerSet makeprg=pandoc\ --standalone' .
+      \ '\ --metadata\ title=%:t:r:S' .
+      \ '\ --metadata\ lang=' . matchstr(&spelllang, '^\a\a') .
+      \ '\ --from=' . s:PandocFiletype(&filetype) .
+      \ '\ --output\ %:r:S.$*\ %:S'
+
+CompilerSet errorformat="%f",\ line\ %l:\ %m
+
+let &cpo = s:keepcpo
+unlet s:keepcpo