Handle syntax without syntax or indent files

A syntax in the config without a syntax file, like postcss, showed
E484 on opening a file. Skip loading it, with a warning in debug.

A syntax without an indent file used cindent, which broke indent of
languages like pug, stylus without their plugins, or toml. Keep the
indent instead. Lines outside blocks still use cindent.

test/cases/no-syntax-indent-file.vim

@@ -0,0 +1,15 @@
+" postcss has no syntax or indent file, toml has no indent file. They have
+" their blocks and keep indent
+let g:vim_vue_plugin_config = {
+      \'syntax': {
+      \   'template': ['html'],
+      \   'style': ['css', 'postcss'],
+      \   'i18n': ['toml'],
+      \},
+      \}
+
+function! Check()
+  call ExpectBlock('color: blue', 'style', 'postcss', 'postcssStyleBlock')
+  call ExpectBlock('hello', 'i18n', 'toml', 'tomlI18nBlock')
+  call ExpectIndentEachLine()
+endfunction

test/cases/no-syntax-indent-file.vue

@@ -0,0 +1,15 @@
+<template>
+  <div>x</div>
+</template>
+
+<style lang="postcss">
+p
+  color: red
+  a
+    color: blue
+</style>
+
+<i18n lang="toml">
+[en]
+  hello = "hi"
+</i18n>