Allow non-contributors to take an issue with .take

[carreter]

Changes in this PR

Adds a GitHub action that allows non-contributors to self-assign issues when they include ".take" in an issue comment.

Why are you making these changes?

Allows for non-contributors to indicate they are working on an issue. Part of #346 .

Are any changes breaking? (IMPORTANT)

No.

Pre-merge checklist

All of these must be satisfied before this PR is considered
ready for merging. Mergeable PRs will be prioritized for review.

• New packages/exported functions have docstrings.
• New/changed functionality is thoroughly tested.
• New/changed functionality has a function giving an example of its usage in the associated test file. See primers/primers_test.go for what this might look like.
• Changes are documented in CHANGELOG.md in the [Unreleased] section.
• All code is properly formatted and linted.
• The PR template is filled out.