Add an icon for the "Paste without formatting" context menu item

### Is your feature request related to a problem?

The "Paste without formatting" context menu item has no icon even if the `userChrome.icon.menu.full` is enabled.

If you can't make the mentioned item show, then try going to this website and right click on the main text area:
- https://onlinenotepad.org/notepad

### The solution I'd like

Add an appropriate icon for the "Paste without formatting" context menu item, but make it show only when the userChrome.icon.menu.full is enabled.

The only problem would be where such an icon is in default places, so a custom-made icon would be good since Firefox UI Fix has had custom-made icons in recent months.

### Screenshots or video


### Alternatives I've considered

_No response_

### Additional context

_No response_

Reproduce the missing icon by enabling userChrome.icon.menu.full and right-clicking the main text area at https://onlinenotepad.org/notepad. Then inspect the existing Firefox UI Fix icon rules and determine how the "Paste without formatting" context-menu item is targeted; done means an appropriate icon appears only when that setting is enabled.

Written by the indexing model from the issue text.