Add a PR template for GitHub

[0xEAB]

Feedback welcome.

Docs: https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository

[dlang-bot]

Thanks for your pull request, @0xEAB!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10975"

[0xEAB]

@jmdavis pointed out that we should figure out how to inject the commit message into the pre-provided “placeholder” text. Otherwise this would be annoying.

[0xEAB]

Jacob Carlborg wrote on Discord:

on GitHub it will first include the commit message (assuming there's only one commit) then the template will follow in the description. So I just remove the template and leave the commit message as is.