feat: extend logic to Linkedin

[msramalho]

To do so, replicate the parsing logic implemented in https://ollie-boyd.github.io/Linkedin-post-timestamp-extractor/ and add the necessary changes to:

• manifest.json - new content script for tiktok
• linkedin.js parser file
• if relevant see also #4
• update the README accordingly

any additional css/js changes.

Please include screenshots/example URLs in any PR.