fix(strikethrough): update strikethrough shortcut

[bdbch]

Please describe your changes

This PR changes the shortcut for strikethrough to a more standardized version similar to other apps (Notion, Dropbox Paper).

Now the default shortcut is Mod+Shift+s for MacOS and Ctrl+Shift+s for other operation systems.

See https://defkey.com/search?irq=strikethrough

How did you accomplish your changes

Changed the addKeyboardShortcuts function

How have you tested your changes

Tested them locally on MacOS and Windows.

How can we verify your changes

Check the deployed demo

Remarks

nothing

Checklist

• The changes are not breaking the editor
• Added tests where possible
• Followed the guidelines
• Fixed linting issues

Related issues

closes #2540

[netlify]

✅ Deploy Preview for tiptap-embed ready!

Name	Link
🔨 Latest commit	ec0b4a3
🔍 Latest deploy log	https://app.netlify.com/sites/tiptap-embed/deploys/64cd997fbaba0000087b5a0c
😎 Deploy Preview	https://deploy-preview-4288--tiptap-embed.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[Nantris]

This PR needs to be amended to be case-insensitive like the others (eg bold) This apparently isn't necessary for hotkeys which incorporate Shift

---

Also the docs seem to have not been updated to reflect the key change: https://tiptap.dev/api/marks/strike#keyboard-shortcuts