Feature request: tab-line-new-button-show

[No Wayman]

Juri very kindly added `tab-bar-new-button-show' to control the 
display of the new button in tab-bar-mode in response to a message 
I sent him on Reddit.

I think an analogue variable in tab-line-mode would be great for 
keyboard-centric users of tab-line-mode.

Thanks for all the hard work,
Nick Vollmer