Support Vue3

[nichtich]

Feature request

What problem does this feature solve?

Docsify does not work with Vue3 because in Vue3 there is no global app. This results in error

Uncaught TypeError: window.Vue is not a constructor

What does the proposed API look like?

When using Vue3, you have to manually initialize a Vue instance because no default instance is created.

It would also be possible to create a default instance with Vue3 but I need to configure my own instance anyway.

How should this be implemented in your opinion?

Add an additional condition Vue.version.split(".")[0]<=2 and don't call Vue().$mount if it is false.

Are you willing to work on this yourself?

I can provide a pull request if the proposed solution is ok to you.

[jhildenbiddle]

Address in #1271. Waiting for review and merge.

[jhildenbiddle]

@nichtich --

Added in #1271. Updated docs are available here: https://github.com/docsifyjs/docsify/blob/develop/docs/vue.md