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.

test/cases/shiftwidth-zero.vim

@@ -0,0 +1,11 @@
+" shiftwidth=0 uses the value of tabstop, same as shiftwidth=4 here
+set shiftwidth=0 tabstop=4
+let g:vim_vue_plugin_config = {'foldexpr': 1}
+let g:test_flatten = 1
+
+function! Check()
+  let levels = [1, 2, 3, 2, 3, 4, 3, 3, 2, 1]
+  for lnum in range(1, len(levels))
+    call ExpectFoldlevel(lnum, levels[lnum - 1])
+  endfor
+endfunction