chore: Add issue templates to the repo

.github/ISSUE_TEMPLATE/bug-report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+---
+
+- [ ] Check if updating to the latest version resolves the issue
+
+**Environment**
+- [ ] I am using `@preact/signals-core`
+- [ ] I am using `@preact/signals`
+- [ ] I am using `@preact/signals-react`
+  - [ ] I understand usage changed in v2, and I've followed the [React Integration instructions](https://github.com/preactjs/signals/tree/chore/issue-templates/packages/react#react-integration)
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Please provide a link to a StackBlitz/CodeSandbox/Codepen project or a GitHub repository that demonstrates the issue.
+
+Steps to reproduce the behavior:
+
+1. Go to '...'
+2. Click on '....'
+3. See error
+
+**Expected behavior**
+What should have happened when following the steps above?

.github/ISSUE_TEMPLATE/feature-request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: feature request
+assignees: ''
+---
+
+**Describe the feature you'd love to see**
+A clear and concise description of what you'd love to see added to Preact Signals.
+
+**Additional context (optional)**
+Add any other context or screenshots about the feature request here.