Improve our default new Issue template

Overview

Our default new Issue template was last updated quite a while ago and could use a little restructuring to ensure we're capturing as much information as possible to assist.

Proposal

New content

# Overview

<!-- Details of the issue.  Include any console output or screenshots. -->

# Steps to replicate (optional)

<!-- Clear steps of how to replicate the issue. -->

# Proposal (optional)

<!-- Description of any proposal you might have. -->

# Environment (optional)

- Operating system: macOS/Linux/etc
- The contents of your `gdk.yml`
- Ruby version: `<!-- output of `ruby --version` -->`
- GDK version: `<!-- output of `git rev-parse --short HEAD` -->`

Diff

Using the diff formatter doesn't seem to yield useful output here, so I went with a visual diff: