Add GitHub templates

Author: danielbachhuber

.github/ISSUE_TEMPLATE

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+<!--
+
+Thanks for creating a new issue!
+
+Found a bug or want to suggest an enhancement? Before completing your issue, please review our best practices: https://make.wordpress.org/cli/handbook/bug-reports/
+
+Need help with something? GitHub issues aren't for general support questions, but there are other venues you can try: https://wp-cli.org/#support
+
+You can safely delete this comment.
+
+-->

.github/PULL_REQUEST_TEMPLATE

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+<!--
+
+Thanks for submitting a pull request!
+
+Here's an overview to our process:
+
+1. One of the project committers will soon provide a code review.
+2. You are expected to address the code review comments in a timely manner.
+3. Please make sure to include functional tests for your changes.
+4. The reviewing committer will merge your pull request as soon as it passes code review (and provided it fits within the scope of the project).
+
+You can safely delete this comment.
+
+-->