Add issue templates (#519)

Author: mosuem

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Add a minimal working example or, if not possible or available, any code which might help to reproduce the problem
+```
+void main(List<String> arguments) {
+}
+```
+
+**System info**
+<The output of `dart pub deps --style compact` in your project>

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/ISSUE_TEMPLATE/missing-locale.md

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+---
+name: Missing Locale
+about: My favorite locale is missing
+title: ''
+labels: missing-locale-data
+assignees: ''
+
+---
+
+**Which locale is missing?**
+<e.g. `en_US`>
+
+**Which locale have you considered of using instead, but was not sufficient?**
+<e.g. `en_NZ`>

.github/ISSUE_TEMPLATE/wrong-locale-data.md

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+---
+name: Wrong Locale Data
+about: I think the locale data is wrong
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+Is it really wrong?
+
+For locale data, there are only a few hard and fast facts, such as the names of days and months. Even these are subject to regional variants, and to changes over time. For example, the “Czech Republic” is known since 2016 as “Czechia”.
+
+For most of the data, there is no single right answer, but rather a judgment call on what is more customary/appropriate/convenient/whatever.
+
+Most locale data depends on opinion, preference, and compromise.
+
+If you do feel strongly that the data is truly wrong, or can be improved significantly, then please report this to [CLDR](http://cldr.unicode.org/index/bug-reports) and not this repository, for which the CLDR data is the source of truth.