A userscript that adds a button to insert a details block into comments

• A button is added to the issue/pull request comment toolbar.
• Clicking the button will add a <details> block with the selected text inside.
• Adding an "open" attribute to the details element will start with the container expanded. A hint using "not-open" is included and is ignored unless changed to "open".
• A blank line is included after the summary to allow markdown elements to render properly inside of the details container (ref).

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