docs: add Dart announcement to README

[jhuleatt]

Added a banner to the main README.md to announce Dart support in Cloud Functions for Firebase. This includes links to the official Firebase blog post announcement and the dart-launch branch containing the Dart samples.

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PR created automatically by Jules for task 6768520190810608194 started by @jhuleatt

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[gemini-code-assist]

Code Review

This pull request updates the README.md to include an announcement regarding Dart support for Cloud Functions for Firebase. The review feedback correctly identifies that this change introduces inconsistencies elsewhere in the documentation, specifically noting that the list of available samples and environment descriptions still only mention Node and Python and should be updated for accuracy.

[gemini-code-assist]

The announcement of Dart support introduces several inconsistencies with the existing documentation in this file that should be addressed for clarity:\n\n* Line 7: The list of available samples still only mentions Node and Python.\n* Line 13: The description of Cloud Functions still defines it as a 'Node.js environment' where you can run 'JavaScript or Python'.\n* Quickstarts: The quickstart sections (starting at line 22) do not include links to the new Dart samples.\n\nPlease update these sections to include Dart to ensure the documentation is accurate and consistent across the entire repository.