chore(dev): add pull_request_template.md for github bug

.github/pull_request_template.md

+<!-- ℹ お読みください / README
+PRありがとうございます！ PRを作成する前に、コントリビューションガイドをご確認ください:
+Thank you for your PR! Before creating a PR, please check the contribution guide:
+https://github.com/misskey-dev/misskey/blob/develop/CONTRIBUTING.md
+-->
+
+## What
+<!-- このPRで何をしたのか？ どう変わるのか？ -->
+<!-- What did you do with this PR? How will it change things? -->
+
+## Why
+<!-- なぜそうするのか？ どういう意図なのか？ 何が困っているのか？ -->
+<!-- Why do you do it? What are your intentions? What is the problem? -->
+
+## Additional info (optional)
+<!-- テスト観点など -->
+<!-- Test perspective, etc -->
+
+## Checklist
+- [ ] Read the [contribution guide](https://github.com/misskey-dev/misskey/blob/develop/CONTRIBUTING.md)
+- [ ] Test working in a local environment
+- [ ] (If needed) Update CHANGELOG.md
+- [ ] (If possible) Add tests