[tool] triage report csv output

[pq]

See #8101.

Adds CSV output to make importing into sheets easier:

☁  flutter-intellij [master] ⚡  dart tool/triage/bin/triage.dart

issues: 743 (+ 6 PRs)
unprioritized: 684 (92%)
created within ...
| 1 week | 2 weeks | 1 month | 3 months | 1 year | 3 years | longer |
| --- | --- | --- | --- | --- | --- | --- |
| 2 | 1 | 0 | 8 | 53 | 182 | 438 |


06/24/2025, 2, 1, 0, 8, 53, 182, 438, , 743, 684

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our [contributor guide](https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Dart contributions to our repos should follow the Dart style guide and use dart format.
• Java and Kotlin contributions should strive to follow Java and Kotlin best practices (discussion).