[Bug report] Debug plugin: It takes effect in all modes

[yzx9]

Bug report

Description

We see in the document that this plugin only take effect in development mode, but in fact it works in any mode.

Steps to reproduce

• Open vuepress.github.io
• The Debug button was shown in the right-bottom

Expected behavior

Debug plugin should work only in development mode.

BTW, why not add an option: devOnly=true

Screenshots

None

Environment info

@vuepress/plugin-debug@2.0.0-alpha.21