feat:add ssr styles inject function.

[lifeishell]

feat: Finish SSR part css inject.
linked to #1759

Exposes style's __inject__ function (which comes from vue-style-loader ) to script
Then you can use it to append styles on SSR context
Usage:

App.__inject__(context); // append styles on req context 
app = Vue.createSSRApp(App, context);
... other ssr codes ...
html = template.replace('</head>', `${context.context || ''}</head>`); // append styles to html head