add GitHub issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a problem with SMAPI.
+
+---
+
+<!--
+
+Only report a bug here if you're sure it's a SMAPI bug! To request support instead, see:
+   - #modding on Discord: https://stardewvalleywiki.com/Modding:Community#Discord
+   - support forum thread: https://community.playstarbound.com/threads/108375
+   - Nexus mod page: https://www.nexusmods.com/stardewvalley/mods/2400
+
+Replace the instructions below with the bug details.
+
+-->
+
+**Describe the bug**
+A clear and concise description of what the bug is. Provide any other details you think might be relevant here.
+
+**To Reproduce**
+Exact steps which reproduce the bug, if possible. For example:
+1. Load save '...'.
+2. Walk to '....'.
+3. Click '....'.
+4. Error occurs.
+
+**Log file**
+Upload your SMAPI log to https://log.smapi.io and post a link here.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for SMAPI.
+
+---
+
+<!--
+
+GitHub issues are only used for development tasks. Please don't submit feature requests here! Instead, see...
+
+   - #modding on Discord: https://stardewvalleywiki.com/Modding:Community#Discord
+   - support forum thread: https://community.playstarbound.com/threads/108375
+   - Nexus page: https://www.nexusmods.com/stardewvalley/mods/2400
+
+-->

.github/ISSUE_TEMPLATE/general.md

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+---
+name: General
+about: Create a ticket about something else.
+
+---
+
+<!--
+
+GitHub issues are only used for development tasks. For support and questions, see...
+
+   - #modding on Discord: https://stardewvalleywiki.com/Modding:Community#Discord
+   - support forum thread: https://community.playstarbound.com/threads/108375
+   - Nexus page: https://www.nexusmods.com/stardewvalley/mods/2400
+
+-->