issue and PR templates

.github/ISSUE_TEMPLATE.md

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+<!--
+Thanks for raising an issue! To help us help you, if you've found a bug please consider the following:
+
+* If you can demonstrate the bug using https://svelte.technology/repl, please do.
+* If that's not possible, we recommend creating a small repo that illustrates the problem.
+
+Reproductions should be small, self-contained, correct examples – http://sscce.org.
+
+Occasionally, this won't be possible, and that's fine – we still appreciate you raising the issue. But please understand that Svelte is run by unpaid volunteers in their free time, and we will prioritise bugs that follow these instructions.
+
+If you have a stack trace to include, we recommend putting inside a `<details>` block for the sake of the thread's readability:
+
+<details>
+  <summary>Stack trace</summary>
+
+  Stack trace goes here...
+</details>
+-->

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thank you for creating a pull request. Before submitting, please note the following:
+
+* If your pull request implements a new feature, please raise an issue to discuss it before sending code. In many cases features are absent for a reason.
+* This message body should clearly illustrate what problems it solves. If there are related issues, remember to reference them.
+* Ideally, include a test that fails without this PR but passes with it. PRs will only be merged once they pass CI. (Remember to `npm run lint`!)
+-->