Support OSC 110, 111, and 112 for resetting colors
This adds support for OSC 110, 111, and 112 escape sequences to reset
the foreground, background, and cursor colors in the terminal. The
changes include handling these sequences in the `strhandle` function of
`st.c`, allowing applications to reset colors to their default values.
The OSC sequences originated from Xterm control sequences and are now
widely used in terminal applications and supported by many terminal
emulators. For applications, this allows them to reset colors to
default values without needing to know the colors beforehand.