feat: support debug in dev process (#1051)

'* + Enter' to log the keys of AppContext, while 'key + Enter' is to log the value of corresponding
field in AppContext

packages/@vuepress/core/lib/dev.js

@@ -79,6 +79,17 @@ module.exports = async function dev (sourceDir, cliOptions = {}) {
   const port = await resolvePort(cliOptions.port || ctx.siteConfig.port)
   const { host, displayHost } = await resolveHost(cliOptions.host || ctx.siteConfig.host)
 
+  // debug in a running dev process.
+  process.stdout.on('data', chunk => {
+    const parsed = chunk.toString('utf-8').trim()
+    if (parsed === '*') {
+      console.log(Object.keys(ctx))
+    }
+    if (ctx[parsed]) {
+      console.log(ctx[parsed])
+    }
+  })
+
   config
     .plugin('vuepress-log')
     .use(DevLogPlugin, [{