fix($core): watch file changes event (fix #2392)

[billyyyyy3320]

Summary

Fix #2392
Relevant PR: #2409

cc @sodatea

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Other information:

[haoqunjiang]

With this fix, the dev configuration is reloaded and the page refreshes, but somehow the new build still does not take the changed file into account.

Ideally, the configuration does not need to be reloaded, only a webpack rebuild needs to be triggered and the page should be hot updated by webpack, not by vuepress.