feat(build): allow ignoring dead links

[brc-dd]

This PR introduces a new option ignoreDeadLinks in user config:

// .vitepress/config.js

export default {
  ignoreDeadLinks: true // <-- this will still warn users on dead links but won't throw error
                        // default value is false, i.e. builds will fail on encountering dead links
}

Fixes #586.