Add username support for LinkedIn

[TedTschopp]

LinkedIn vanity URL validation

Category: social

Goal: Check availability of LinkedIn public profile URLs (vanity names).

Notes:

• LinkedIn usernames typically appear in URLs like https://www.linkedin.com/in/{username}/.
• LinkedIn may return 999/redirects or require careful handling of headers.

Implementation suggestions:

• Investigate the behavior of https://www.linkedin.com/in/{username}/ for existing vs non-existing users.
• If a clean 404 is not available, use generic_validate() and look for strong indicators in response.text.

Implementation notes:

• Add social/linkedin.py with validate_linkedin(username: str).
• Document any rate limit / bot-detection quirks in comments.