Added issue/PR templates (#385)

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: "\U0001F41B Bug report"
+about: Create a report to help us improve
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+<!--
+Thank you for reporting a bug in Boa! This will make us improve the engine. But first, fill the following template so that we better understand what's happening. Feel free to add or remove sections as you feel appropriate.
+-->
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+<!-- E.g.:
+The variable statement is not working as expected, it always adds 10 when assigning a number to a variable"
+-->
+
+**To Reproduce**
+Steps to reproduce the issue, or JavaScript code that causes this failure.
+
+<!-- E.g.:
+This JavaScript code reproduces the issue:
+```javascript
+var a = 10;
+a;
+```
+-->
+
+**Expected behavior**
+Explain what you expected to happen, and what is happening instead.
+
+<!-- E.g.:
+Running this code, `a` should be set to `10` and printed, but `a` is instead set to `20`. The expected behaviour can be found in the [ECMAScript specification][spec].
+
+[spec]: https://www.ecma-international.org/ecma-262/10.0/index.html#sec-variable-statement-runtime-semantics-evaluation
+-->
+
+**Build environment (please complete the following information):**
+ - OS: [e.g. Fedora Linux]
+ - Version: [e.g. 32]
+ - Target triple: [e.g. x86_64-unknown-linux-gnu]
+ - Rustc version: [e.g. rustc 1.43.0 (4fb7144ed 2020-04-20), running `rustc -V`]
+
+**Additional context**
+Add any other context about the problem here.
+
+<!-- E.g.:
+You can find more information in [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var).
+-->

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Discord channel
+    url: https://discord.gg/tUFFk9Y
+    about: Please ask and answer questions here.

.github/ISSUE_TEMPLATE/custom.md

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+---
+name: Custom
+about: Open an issue in the repo that is neither a bug or a feature
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!--
+Thank you for contributing to Boa! Please, let us know how can we help you.
+-->
+E.g.: I think we should improve the way the JavaScript interpreter works by...

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: "\U0001F680 Feature request"
+about: Suggest an idea for this project
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+<!--
+Thank you for adding a feature request to Boa! As this is an experimental JavaScript engine, there will probably be many ECMAScript features left to implement. In order to understand the feature request as best as possible, please fill the following template. Feel free to add or remove sections as needed.
+-->
+
+**ECMASCript feature**
+Explain the ECMAScript feature that you'd like to see implemented.
+
+<!-- E.g.:
+I would like to see `switch` statement parsing and execution implemented. [ECMAScript specification][spec].
+
+[spec]: https://www.ecma-international.org/ecma-262/10.0/index.html#sec-switch-statement
+-->
+
+**Example code**
+Give a code example that should work after the implementation of this feature.
+
+<!-- E.g.:
+This code should now work and give the expected result:
+```javascript
+let a = "hello";
+let b;
+switch (a) {
+    case 'hello':
+        b = 'world';
+        break;
+    case 'world':
+        b = 'hello';
+        break;
+    default:
+        b = 'hello world';
+}
+b;
+```
+The expected output is `world`.
+-->

.github/PULL_REQUEST_TEMPLATE.md

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+<!---
+Thank you for contributing to Boa! Please fill out the template below, and remove or add any
+information as you feel neccesary.
+--->
+
+This Pull Request fixes/closes #{issue_num}.
+
+It changes the following:
+ - 
+ - 
+ -