feat: Add copy/paste buttons to the toolbar in browsers that support it

[personalizedrefrigerator]

Summary

Adds a copy/paste button to the selection toolbar menu. This allows mobile devices to copy and paste content from the system clipboard.

To-do

• [ ] UI design -- the current UI makes it difficult to access paste when content is selected. It might instead make sense to display copy and paste buttons side by side. Alternatively, the copy button could be shown in a menu that floats above selected content.
• [ ] Testing

Screenshots/other information

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 6b4f8a8):

https://js-draw--pr62-work-add-copy-paste-b4bb6pd7.web.app

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[personalizedrefrigerator]

Closing in favor of https://github.com/personalizedrefrigerator/js-draw/pull/78.