feat($markdown): markdown slot shortcut

packages/@vuepress/markdown/lib/contentSlotsContainers.js

+const container = require('markdown-it-container')
+
+const SLOT_KEY = 'slot'
+
+module.exports = md => {
+  md
+    .use(container, SLOT_KEY, {
+      render: (tokens, idx) => tokens[idx].nesting === 1
+        ? `<template slot="${tokens[idx].info.trim().slice(SLOT_KEY.length).trim()}">`
+        : '</template>'
+    })
+}

packages/@vuepress/markdown/lib/index.js

@@ -13,6 +13,7 @@ const componentPlugin = require('./component')
 const hoistScriptStylePlugin = require('./hoist')
 const convertRouterLinkPlugin = require('./link')
 const containersPlugin = require('./containers')
+const contentSlotsContainersPlugin = require('./contentSlotsContainers')
 const snippetPlugin = require('./snippet')
 const emojiPlugin = require('markdown-it-emoji')
 const anchorPlugin = require('markdown-it-anchor')
@@ -75,6 +76,10 @@ module.exports = ({
       .use(containersPlugin)
       .end()
 
+    .plugin('content-slots-containers')
+      .use(contentSlotsContainersPlugin)
+      .end()
+
     .plugin('emoji')
       .use(emojiPlugin)
       .end()