Add issue and PR templates

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: 🐛 Bug report
+about: Report a bug to help us improve Viper
+---
+<!--
+Thank you for sending a bug report! Here are some tips:
+
+1. Please fill out the template below to make it easier to debug your problem.
+2. If you are not sure if it is a bug or not, you can ask in the Gophers slack channel `#viper`.
+-->
+
+**Expected behavior (what you expected to happen)**:
+
+**Actual behavior (what actually happened)**:
+
+**Repl.it link**:
+<!-- you can use the following example: https://repl.it/@sagikazarmark/Viper-example -->
+
+**Code reproducing the issue**:
+<!-- just to make sure the content of the repl doesn't get lost -->
+
+```go
+
+```
+
+<!-- include configuration file example if necessary -->
+
+**Environment**:
+- Viper version:
+- Config source: <!-- flag, env, file, etc -->
+- File format: <!-- JSON, YAML, TOML, etc -->
+
+**Anything else we should know?**:

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: 📓 Reference
+    url: https://pkg.go.dev/mod/github.com/spf13/viper
+    about: Check the Go code reference
+
+  - name: 💬 Slack channel
+    url: https://gophers.slack.com/messages/viper
+    about: Please ask and answer questions here

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: 🚀 Feature request
+about: Suggest an idea for Viper
+---
+<!--
+ Thank you for sending a feature request!
+ Please describe what you would like to change/add and why in detail by filling out the template below.
+ -->
+
+**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 to see**
+<!-- A clear and concise description of what would you like 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/PULL_REQUEST_TEMPLATES.md

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+<!--
+Thank you for sending a pull request! Here some tips for contributors:
+
+1. Fill the description template below.
+2. Include appropriate tests (if necessary). Make sure that all CI checks passed.
+3. If the Pull Request is a work in progress, make use of GitHub's "Draft PR" feature and mark it as such.
+-->
+
+**Overview**:
+<!-- Describe your changes briefly here. -->
+
+**What problem does it solve?**:
+<!--
+- Please state in detail why we need this PR and what it solves.
+- If your PR closes some of the existing issues, please add links to them here.
+  Mentioned issues will be automatically closed.
+  Usage: "Closes #<issue number>", or "Closes (paste link of issue)"
+-->
+
+**Special notes for a reviewer**: