Fix: Using `Clipboard API` instead of `execCommand`

[NI57721]

On Chrome version 105.0.5195.102 (64-bit), execCommand("Paste") no longer seems to work. https://github.com/philc/vimium/issues/4120

What is worse, execCommand is said completely not to work by January 2023, since Manifest V2 is supposed to stop working. https://www.bleepingcomputer.com/news/google/google-manifest-v2-chrome-extensions-to-stop-working-in-2023/

Instead of execCommand, I use Clipboard API, which is designed to supersede accessing the clipboard using document.execCommand().

Thanks.