Add issue and pr templates that warn about common problems

Author: marijnh

.github/ISSUE_TEMPLATE.md

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+<!--
+Do not use this bug tracker for questions. We have a forum (https://discuss.codemirror.net) for those.
+
+In order to get us to look at something, you need to make it easy for us to understand what you are doing and what went wrong. When possible, a code example that demonstrates the problem helps a lot. If there's any chance at all that a problem is related to the browser, include information about which browser(s) you tested with.
+-->

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+NOTE: We are not accepting pull requests for new modes or addons. Please put such code in a separate repository, and release them as stand-alone npm packages. See for example the [Elixir mode](https://github.com/ianwalter/codemirror-mode-elixir).
+
+Also pull requests that rewrite big chunks of code or adjust code style to your own taste are generally not welcome. Make your changes in focused steps that fix or improve a specific thing.
+-->