Added issue templates (#1805)

This adds issue templates for the prism project after #1802.

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Information:**
+- Prism version: [e.g. 1.14, latest from the download page, etc.]
+- Environment: [e.g. Browser, Node, Webpack]
+
+Does the latest version of Prism from the [download page](https://prismjs.com/download.html) also have this issue?
+
+If you're using Prism with Node or Webpack, please also see the [usage section](https://prismjs.com/index.html#basic-usage).
+
+**Description**
+A clear and concise description of what the bug is and how to reproduce it.
+If applicable, add screenshots to help explain your problem.
+
+**Example**
+An example which reproduces the bug. This can be a link to a website, a [JSFiddle](https://jsfiddle.net/), an attached example project, etc.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest a new feature for Prism.
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+**Motivation**
+Please explain the problem you're having/why you propose this feature. Ex. I'm always frustrated when [...]
+
+**Description**
+A clear and concise description of what you want to happen.
+
+**Alternatives**
+A clear and concise description of any alternative solutions or features you've considered, if applicable.

.github/ISSUE_TEMPLATE/highlighting-bug-report.md

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+---
+name: Highlighting bug report
+about: Does Prism highlight parts of your code incorrectly or is a language feature
+  not supported?
+title: ''
+labels: language-definitions
+assignees: ''
+
+---
+
+**Information**
+- Language: [e.g. C++, JavaScript, YAML, etc.]
+- Plugins: [what plugin you are using]
+
+Does the problem still occur in the latest version of Prism? You can check using the [test page](https://prismjs.com/test.html) or get the latest version at the [download page](https://prismjs.com/download.html).
+
+**Description**
+A clear and concise description of what is being highlighted incorrectly and how it should be highlighted instead. Add screenshots to illustrate the problem.
+
+**Code snippet**
+
+<details>
+<summary>The code being highlighted incorrectly.</summary>
+
+```
+Your code goes here
+```
+
+</details>

.github/ISSUE_TEMPLATE/new-language-request.md

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+---
+name: New language request
+about: Suggest a new language Prism should support.
+title: ''
+labels: language-definitions, new langauge
+assignees: ''
+
+---
+
+**Language**
+A short description of the language.
+
+**Additional resources**
+E.g. The official website, documentation or language specification.