[tc_terminal] Add `TCTERM_{FG,BG}_DEFAULT` for default color

[darkuranium]

Windows and POSIX handle colors differently --- in POSIX, one can use \e[39m (foreground) or \e[0m (both) to reset the colors. For that purpose, TCTERM_FG_DEFAULT and TCTERM_BG_DEFAULT should be added, which resolve to some system-specific "default" color.

Perhaps TCTERM_{FG,BG}_STARTING should also be added to specify the selected color at program start (which is distinct from the default color of the terminal).