Add Github issue templates

.github/ISSUE_TEMPLATE/blank-issue.md

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+---
+name: Blank issue
+about: Create an issue from scratch
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create an issue about the extension's behavior or appearance
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Description**
+(Describe what the issue is, if it is reproducible, and if possible, how to reproduce it.)
+
+**Browser version**
+(e.g. Firefox 83.0)
+
+**Yomichan version**
+(e.g. 20.11.1.1)
+
+**Exported settings file**
+(If you think it might be relevant, create a settings export file using the _Export Settings_ button on the settings page; zip or rename the resulting file to .txt to attach to the issue.)

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Create an issue suggesting a new feature or change
+title: "[Feature Request]"
+labels: ''
+assignees: ''
+
+---
+
+(Describe the feature or change)