Use npm audit by default, keep Snyk optional

[julian-michelmann]

Right now the action defaults to Snyk for node projects, which requires a token and license.

Change

• Default to npm audit for Node.js projects.
• Only use Snyk if SNYK_TOKEN is set in the workflow.

Why

• npm audit works out of the box for everyone.
• Snyk remains available for teams that prefer it.

Tasks

• Implement a tool that is using nmpt-audit.
• Make the Snyk tool use conditional if the SNYK_TOKEN is set.
• Adjust the prompts if necessary.
• Adjust the documentation.