Reorganize GitHub Workflows to enable running on PRs

[Lezurex]

This PR reorganizes the workflows a bit and splits them into smaller parts. This is to achieve running some workflows on PRs created from fork branches.

• Code scanning (CodeQL and nodejsscan) now runs separately and on fork PRs as well.
• Tests have their own workflow now and can run on fork PRs now as well.
• The mirror and flathub pushes run only after a test workflow has succeeded on the main branch.
• Snyk Scanning runs on repo push only, not in fork PRs, as it requires a token.

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.