Added issue, pull request templates. (#2869)

* docs(meta): Added issue, pull request templates. 📜 [ci skip]

* Addressed PR#2869 comments [ci skip]

* Addressed some more comments on PR#2869 [skip ci]

.github/ISSUE_TEMPLATE.md

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+<!--
+Have you read Mocha's Code of Conduct? By filing an Issue, you are expected to comply with it, including treating everyone with respect: https://github.com/mochajs/mocha/blob/master/.github/CODE_OF_CONDUCT.md
+For more, check out the Mocha Gitter chat room: https://gitter.im/mochajs/mocha
+-->
+
+### Prerequisites
+<!--
+Place an `x` between the square brackets on the lines below for every satisified prerequisite.
+-->
+* [ ] Checked that your issue isn't already filed by cross referencing [issues with the `common mistake` label](https://github.com/mochajs/mocha/issues?utf8=%E2%9C%93&q=is%3Aissue%20label%3Acommon-mistake%20)
+* [ ] Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
+* [ ] 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
+* [ ] Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with:
+`node node_modules/.bin/mocha --version`(Local) and `mocha --version`(Global). We recommend avoiding the use of globally installed Mocha.
+
+### Description
+<!--
+[Description of the issue]
+-->
+
+### Steps to Reproduce
+
+<!--
+Please add a series of steps to reproduce the problem. See https://stackoverflow.com/help/mcve for in depth information
+on how to create a mimimal, complete, and verifiable example.
+-->
+
+**Expected behavior:** [What you expect to happen]
+
+**Actual behavior:** [What actually happens]
+
+**Reproduces how often:** [What percentage of the time does it reproduce?]
+
+### Versions
+<!--
+ If applicable, please specify:
+ * The output of `mocha --version` and `node node_modules/.bin/mocha --version`:
+ * The output of `node --version`:
+ * The version and architecture of your operating system:
+ * Your shell (bash, zsh, PowerShell, cmd, etc.):
+ * Your browser and version (if running browser tests):
+ * Any other third party Mocha related modules (with versions):
+ * The code transpiler being used:
+-->
+
+### Additional Information
+<!--
+Any additional information, configuration or data that might be necessary to reproduce the issue.
+-->

.github/PULL_REQUEST_TEMPLATE.md

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+### Requirements
+
+* Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
+* All new code requires tests to ensure against regressions.
+
+### Description of the Change
+
+<!--
+
+We must be able to understand the design of your change from this description. Keep in mind that the maintainers and/or community members reviewing this PR may not be familiar with the subsystem. Please be verbose.
+
+-->
+
+### Alternate Designs
+
+<!-- Explain what other alternates were considered and why the proposed version was selected -->
+
+### Why should this be in core?
+
+<!-- Explain why this functionality should be in mocha as opposed to it's own package -->
+
+### Benefits
+
+<!-- What benefits will be realized by the code change? -->
+
+### Possible Drawbacks
+
+<!-- What are the possible side-effects or negative impacts of the code change? -->
+
+### Applicable issues
+
+<!--
+* Enter any applicable Issues here.
+
+* Mocha follows semantic versioning: http://semver.org
+
+* Is this a breaking change (major release)?
+* Is it an enhancement (minor release)?
+* Is it a bug fix, or does it not impact production code (patch release)?
+-->