Add codeowners and basic PR template

.github/CODEOWNERS

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+@fastly/devex-engineering

.github/PULL_REQUEST_TEMPLATE.md

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+ All Submissions:
+
+* [ ] Have you followed the guidelines in our Contributing document?
+* [ ] Have you checked to ensure there aren't other open [Pull Requests](https://github.com/fastly/cli/pulls) for the same update/change?
+
+<!-- You can erase any parts of this template not applicable to your Pull Request. -->
+
+### New Feature Submissions:
+
+* [ ] Does your submission pass tests?
+
+### Changes to Core Features:
+
+* [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
+* [ ] Have you written new tests for your core changes, as applicable?
+* [ ] Have you successfully run tests with your changes locally?
+
+### User Impact
+
+* [ ] What is the user impact of this change?
+
+### Are there any considerations that need to be addressed for release?
+
+<!-- Any breaking changes, etc -->