Add GitHub-based plugin update checker

[ilicfilip]

Closes #22

Summary

• Adds plugin-update-checker via Composer to enable automatic update notifications in wp-admin
• The checker points to the GitHub repo and will compare the local plugin version against GitHub Releases

How to test

1. Merge this PR
2. Create a GitHub Release with a tag higher than the current plugin version (e.g., 1.0.1)
3. Go to wp-admin > Plugins or wp-admin > Dashboard > Updates
4. You should see an update notification for Markdown Alternate

Alternatively, to test without creating a real release:

1. Temporarily change the Version header in markdown-alternate.php and the MARKDOWN_ALTERNATE_VERSION constant to 0.0.1
2. Create a GitHub Release tagged 1.0.0
3. Check wp-admin for the update notification
4. Revert the version change after testing

[ilicfilip]

@jdevalk , set this PR as "Ready for review" if you want it.