Fix E409 when css syntax isn't loaded

The v-bind() region in style contains cssValue.* groups, which don't
exist without css syntax, like when syntax in the config has only
script. Opening a file then showed E409 and E475. Add the style
highlights only if css syntax is loaded.

test/cases/no-css.vim

@@ -0,0 +1,7 @@
+" Without css syntax, like only script in config
+let g:vim_vue_plugin_config = {'syntax': {'script': ['javascript']}}
+let g:test_flatten = 1
+
+function! Check()
+  call ExpectEqual(GetVueTag(2), 'script', 'tag')
+endfunction