Enhancement: action-test-results-to-slack could support xunit xml

### Impacted plugin

[None / Other](https://github.com/Automattic/action-test-results-to-slack)

### What

I would like action-test-results-to-slack to support XUNIT XML

### How

Most all test frameworks these days can output XUNIT XML. This plugin would perhaps receive the attention it deserves if that feature was added. I am actually using Playwright myself, but I am using Mocha as my runner and not Playwright/Test so my output format does not match what the plugin handles.

Personally, I would not expect attachments to be handled, just the pass/fail test case status if that is supported with the playwright integration.

Start by locating the action-test-results-to-slack plugin and its existing Playwright result-handling entry point. Review how it extracts pass/fail test-case status, then define completion as accepting XUNIT XML from Mocha or other runners while omitting attachments, with coverage for the supported result data.

Written by the indexing model from the issue text.