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

[tgummerer]

Just a heads up, this is not going to be as simple as it looks at first glance. GitHub Actions uses the pulumi automation api under the hood. destroy does support a "preview only" flag, but I believe it's deprecated because it does not work correctly.

Although on second look, we don't even seem to have implemented previewDestroy in the NodeJS automation API yet. @i-am-tom pulumi/pulumi#19948 seems to hint that we should implement previewDestroy for the NodeJS automation API as well, but it doesn't look like we ever did that. Was that an oversight, or does the preview_only flag work for destroy in the automation API?

[scjody]

Thanks for the information! At this point I've worked around the issue because we have a major project due in early September, but I hope to have time to work on this (manually) later next month.