Replace dynamic subshell syntax bit with a setting

vim-just/syntax/just.vim

@@ -1,77 +1,18 @@
 " Vim syntax file
 " Language: Just Command Runner
-" Maintainer: Dan Buch
-" Latest Revision: 22 May 2021
+" Maintainer: Dan Buch <dan@meatballhat.com>
+" Latest Revision: 28 May 2021
 
 if exists("b:current_syntax")
   finish
 endif
 
-let b:shell_name = "sh"
-let b:load_time_cursor = getpos('.')
-let b:debug = $JUST_VIM_DEBUG == 'enabled'
-
-function! s:justGetShellName()
-  let lineno = 0
-
-  while lineno < line("$")
-    let lineno += 1
-    let line = getline(lineno)
-
-    let set_shell_match = "^ *set  *shell  *:"
-    if matchstr(line, set_shell_match) == ""
-      if b:debug
-        echom "line number " . lineno . " does not match '" . set_shell_match . "': '" . line . "'"
-      endif
-
-      continue
-    endif
-
-    if b:debug
-      echom "found matching line number " . lineno . ": " . line
-    endif
-
-    :normal /"<CR>lv/"<CR>h"ay:nohl
-    let shell_name = @a
-
-    if shell_name != ""
-      if b:debug
-        echom "returning shell_name: '" . shell_name . "'"
-      endif
-
-      return shell_name
-    endif
-  endwhile
-
-  return "sh"
-endfunc
-
-function! s:justSetShellName(shell_name)
-  try
-    if b:debug
-      echom "using shell name: '" . a:shell_name . "'"
-    endif
-
-    exe "syn include @setshellsyntax syntax/" . a:shell_name . ".vim"
-
-    let b:shell_name = a:shell_name
-  catch
-    if b:debug
-      echom "oh no: " v:exception
-      echom "falling back to sh syntax"
-    endif
-
-    syn include @setshellsyntax syntax/sh.vim
-  finally
-    syn region justSubshell matchgroup=justBacktick start="\v\`" skip="\v\\\`" end="\v\`" contains=@setshellsyntax
-    syn region justSubshell matchgroup=justBacktick start="\v\`\`\`" skip="\v\\\`" end="\v\`\`\`" contains=@setshellsyntax
-  endtry
-endfunc
-
-call s:justSetShellName(s:justGetShellName())
+let b:shell_syntax = "sh"
+if exists("g:just_shell_syntax")
+  let b:shell_syntax = g:just_shell_syntax
+endif
 
-delfunc s:justSetShellName
-delfunc s:justGetShellName
+exe "syn include @setshellsyntax syntax/" . b:shell_syntax . ".vim"
 
 syn keyword justKeyword alias export if else set