Restore "unread" tab Attribute

[goodusername123]

This restore the "unread" tab Attribute that was controversially removed all the way back in Firefox 61. This Attribute is useful for theming since it allows custom CSS styling for tabs that has been opened in the background but haven't been actually viewed yet.

To test simply add something such as this to userChrome.css:

#tabbrowser-tabs tab[unread="true"]
{
	font-style: italic !important;
	color: red !important;
}

Semi off-topic

The pull request for updating Twemoji to 15.1.0 (pull 3513) is seemly not preset in the `current` branch for Waterfox 6.5.0 if possible please pull this into the current branch as it is still relevant in the context of ESR128.

[goodusername123]

uh It seems like this pull request got kinda messed up from the current branch getting changed is there anything I need to do on my end or are you able to just pull in the one commit that is relevant to this pull request?

[MrAlex94]

bd954c130c27