fix: LRU does not use the "new" operator

[bailnl]

Summary
Why does an LRU Class not use the new operator? Caused an error Uncaught TypeError: Class constructor LRU cannot be invoked without 'new'

i use npm install, the lru-cache is 5.x. but vuepress dep is 4.x

I found out why lru-cache became 5.x.
The reason is because @vue/cli-shared-utils uses ^5.1.1

Source location: https://github.com/isaacs/node-lru-cache/blob/master/index.js#L27

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Other information:

[ulivz]

If you use yarn then this issue will not exist. of course, it makes sense to upgrade it.