Hide private repository links from Discord presence

[fahdfady]

Issue

Currently, the Discord presence shows "View Repo" button that links to a remote git origin that the extenstion sees through VSCode. this exposes private repositories' URLs through their discord activity.

Changes

• Added discord.showPrivateRepositories configuration option (default: false)
• Implemented GitHub API integration to check repository visibility
• Now, repository links are only shown when they're :

1. Public repositories
2. Private repositories when explicitly enabled via config

resolves #1922

[fahdfady]

@iCrawl can you take a look at this PR?