Add issue template

ISSUE_TEMPLATE.md

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+### Environment
+
+* Elixir & Erlang versions (elixir --version):
+* Operating system:
+* Client (editor or IDE):
+
+### Crash report template
+
+_Delete this section if not reporting a crash_
+
+The canonical client for ElixirLS is the vscode-elixir-ls extension for VS Code. If you encountered a crash using another client, please check whether you can reproduce it in VS Code, then provide the following information:
+
+* Create a new Mix project with `mix new empty`, then open that project with VS Code and open an Elixir file. Is your issue reproducible on the empty project?
+* Check the output log by opening `View > Output` and selecting "ElixirLS" in the dropdown. Please include any output that looks relevant. (If ElixirLS isn't in the dropdown, the server failed to launch.)
+* Check the developer console by opening `Help > Toggle Developer Tools` and include any errors that look relevant.