Add preview-only flag support for destroy command

[scjody]

Closes #1275

Note that I vibe coded this and haven't yet reviewed or tested it. I'll mark the PR as ready for review and remove this message once it's ready!

This adds a new preview-only input parameter that allows users to preview what would be destroyed without actually performing the destruction. This is equivalent to running pulumi destroy --preview-only from the CLI.

Changes:

• Added preview-only input parameter to action.yml
• Updated config.ts to handle the new parameter
• Added tests for the new functionality
• Updated README.md documentation