Add tooltips for format buttons

[atsixian]

Is your feature request related to a problem? Please describe.
We can use shortcuts to format text(e.g. ctrl+b for bold), but the UI doesn't hint this. This knowledge purely depends on the user's experience and common sense.

Describe the solution you'd like
It'll be nice to show a tooltip when hovering on the format button like Notion:

[devrsi0n]

🙌 This is a great idea! I can help you to implement it if you're interested. Here the related code: MarkButton, Popover

[atsixian]

🙌 This is a great idea! I can help you to implement it if you're interested. Here the related code: MarkButton, Popover

Sure! You can assign this to me. I'll work on it.