Support shiftwidth=0 in indent and fold

With shiftwidth=0, Vim uses the value of tabstop, but &sw is 0. So
multiline tags and brackets got wrong indent, and fold levels were wrong
as indent was divided by 0. Use shiftwidth() instead, which is the same
as &sw when it isn't 0.

ftplugin/vue/fold.vim

@@ -88,7 +88,7 @@ endfunction
 
 function! s:IndentLevel(lnum)
   " Add 1 to indentLevel, so start/end tags can fold properly
-  return indent(a:lnum) / &shiftwidth + 1
+  return indent(a:lnum) / shiftwidth() + 1
 endfunction
 "}}}