feat: support debug in dev process

[ulivz]

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

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[spiltcoffee]

I think this PR is causing vuepress dev to crash for me now.

When running it, I get this error:

events.js:167
      throw er; // Unhandled 'error' event
      ^

Error [ERR_STREAM_DESTROYED]: Cannot call write after a stream was destroyed
    at doWrite (_stream_writable.js:406:19)
    at writeOrBuffer (_stream_writable.js:394:5)
    at WriteStream.Writable.write (_stream_writable.js:294:11)
    at clearScreen (node_modules\@vuepress\core\lib\webpack\DevLogPlugin.js:46:18)
    at SyncHook.eval [as call] (eval at create (node_modules\tapable\lib\HookCodeFactory.js:19:10), <anonymous>:7:1)
    at SyncHook.lazyCompileHook (node_modules\tapable\lib\Hook.js:154:20)
    at Watchpack.watcher.compiler.watchFileSystem.watch (node_modules\webpack\lib\Watching.js:139:33)
    at Object.onceWrapper (events.js:273:13)
    at Watchpack.emit (events.js:182:13)
    at Watchpack._onChange (node_modules\watchpack\lib\watchpack.js:118:7)
    at Watchpack.<anonymous> (node_modules\watchpack\lib\watchpack.js:99:8)
    at Watcher.emit (events.js:182:13)
    at node_modules\watchpack\lib\DirectoryWatcher.js:101:9
    at Array.forEach (<anonymous>)
    at DirectoryWatcher.setFileTime (node_modules\watchpack\lib\DirectoryWatcher.js:99:42)
    at DirectoryWatcher.<anonymous> (node_modules\watchpack\lib\DirectoryWatcher.js:312:12)
Emitted 'error' event at:
    at onwriteError (_stream_writable.js:425:12)
    at onwrite (_stream_writable.js:456:5)
    at doWrite (_stream_writable.js:406:11)
    at writeOrBuffer (_stream_writable.js:394:5)
    [... lines matching original stack trace ...]
    at Array.forEach (<anonymous>)

If I comment out the changes made by this PR, then everything works again.

[ulivz]

@spiltcoffee You should provide your context instead of pasting your error here.

[spiltcoffee]

I'll create an example project for ya, see if it's happening outside of my use case.

[spiltcoffee]

@ulivz here you go: https://github.com/spiltcoffee/vuepress-dev-crash

I don't get any additional information or context supplied when the crash occurs.