Add a new c-vt-mouse-encode example that demonstrates how to use the
mouse encoder C API. The example creates a mouse encoder configured
with SGR format and normal tracking mode, sets up terminal geometry
for pixel-to-cell coordinate mapping, and encodes a left button press
event into a terminal escape sequence.
Mirrors the structure of the existing c-vt-key-encode example. Also
adds the corresponding @example doxygen reference in vt.h.
