Support shortcutkeys to save !

`chrome://extensions/shortcuts#:~:text=Save%20for%20later`

↑ Copy the above URL and open it will arrive directly.
this hotkey is Popup, not a specific function , so must add ⌨hotkey for 👉

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- Users will be able to use shortcutkey to directly trigger the function.
- Use the hotkey, it must should 🔔`Chrome.Notifications` !
https://developer.chrome.com/docs/extensions/reference/notifications/

　
　
With the 🔔, users can rest assured when saving with ⌨hotkey!


- ### 　　　　`?` tabs be saved ꘌ see at here👉 https://github.com/allen505/save-for-later/issues/13

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⚠ Of course, this hotkey operating must also support `Save selected tabs` !
In fact, a hotkey will be enough, because you holding the `Shift` on the tab-bar and click 2 times from the beginning to the end, it is equivalent to selecting all the tabs under the window.

- see here 👉 https://github.com/allen505/save-for-later/issues/13