add TERM support for `tmux-256color` and `screen-256color`

[mtfurlan]

Zephyr currently has a hardcoded list of terminal types it supports:

linux,xterm,xterm-color,xterm-256color,vt100

This makes life hard if you use tmux, because forcing tmux to use xterm-256color instead of screen-256color or tmux-256color means the Home and End keys don't work properly, and likely other stuff as I only tested for a day or so.

After some discussion with @stephanosio on discord, the best solution seems to be just to add screen-256color and tmux-256color to the zephyr terminfo database because tmux is a very widely used program.