chore(issues): add issue and PR templates

.github/ISSUE_TEMPLATE.md

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+<!--
+Thank you for raising your concerns, we appreciate your feedback and contributions to this repository.
+
+Before you continue, consider the following:
+
+If you have a "How do I do ...?" question, it is better for you and for us that this question is placed in [StackOverflow](http://stackoverflow.com/questions/tagged/ixjs) or some chat channel. This way, you are making it easier for others to learn from your experiences too.
+
+These "Issues" are meant only for technical problems, bugs, and proposals related to the library.
+
+If your issue is a bug, please follow the format below:
+-->
+
+**IxJS version:**
+
+**Code to reproduce:**
+
+**Expected behavior:**
+
+**Actual behavior:**
+
+**Additional information:**

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thank you very much for your pull request!
+-->
+
+**Description:**
+
+**Related issue (if exists):**