feat: allow external links in sidebar

[brc-dd]

fixes: #205
closes: #481

Config:

import { defineConfig } from 'vitepress'

export default defineConfig({
  themeConfig: {
    sidebar: [
      { items: [{ text: 'External Link', link: 'https://...' }] }
    ]
  }
})

Result:

[kiaking]

Looking good! Made one comment! After that, I'll adjust the "titleless" section style 👍

[brc-dd]

VPLink already adds link class, so I've not duplicated it. Also, I have moved color-related properties to .link so that .icon can use currentColor.