Add issue templates (#1639)

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+What are you trying to achieve?
+
+Have you searched for similar feature requests?
+
+What would you expect the API to look like?
+
+What alternatives have you considered?
+
+Is there a sample image that helps explain?

.github/ISSUE_TEMPLATE/installation.md

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+---
+name: Installation
+about: For help if something went wrong installing sharp
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+What is the output of running `npm install --verbose sharp`?
+
+What is the output of running `npx envinfo --binaries --languages --system --utilities`?
+
+Have you ensured the platform and version of Node.js used for `npm install` is the same as the platform and version of Node.js used at runtime?

.github/ISSUE_TEMPLATE/possible-bug.md

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+---
+name: Possible bug
+about: Please provide steps to reproduce
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+What is the output of running `npx envinfo --binaries --languages --system --utilities`?
+
+What are the steps to reproduce?
+
+What is the expected behaviour?
+
+Are you able to provide a standalone code sample, without other dependencies, that demonstrates this problem?
+
+Are you able to provide a sample image that helps explain the problem?

.github/ISSUE_TEMPLATE/question.md

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+---
+name: Question
+about: For help with an existing feature
+title: ''
+labels: question
+assignees: ''
+
+---
+
+What are you trying to achieve?
+
+Have you searched for similar questions?
+
+Are you able to provide a standalone code sample that demonstrates this question?
+
+Are you able to provide a sample image that helps explain the question?