feat: Sidebar toggle.

[MacZel]

Summary

What kind of change does this PR introduce?

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

Does this PR introduce a breaking change?

• Yes
• No

Tested in the following browsers:

• Chrome
• Firefox
• Safari
• Edge
• IE

Other information:
This change enables to toggle sidebar when in desktop mode. It also styles default scrollbars in chrome and safari.
Default label is 'Hide/Show sidebar'. To change it modify config.js to:

sidebar: {
    toggle: {
        hideText: 'Ukryj menu',
        showText: 'Pokaż menu'
    },
}

[ulivz]

Sorry but we don't plan to add this feature in default theme. maybe you can create a "derived theme" for it.

ref: https://v1.vuepress.vuejs.org/theme/inheritance.html