add GitHub templates for PRs and releases

[hughrun]

This PR adds:

1. a release template for GitHub releases.
2. a pull request template to help contributors and maintainers to understand and merge PRs faster, and label them correctly.

When using GitHub automated release notes, PRs will be split into sections based on the following labels:

• breaking-change or config-change
• dependencies
• enhancement
• fix or bug
• plumbing, tests or deployment
• all other PRs

Any labels not currently in use will be added once this PR is finalised and merged.

Note that PRs can only be in one category and will be placed in the order they are identified (i.e. a PR labelled both enhancement and config-change will appear in the first category (breaking-change or config-change).

[mouse-reeve]

Thank you so much for writing this, I think it will make the release process way easier for everyone. The PR template looks good to me and it can be adjusted as we go if any part of it isn't working for people