add playerspeed cubescript var

[sauerbraten]

the code already exists here:

• https://github.com/sauerbraten/p1xbraten/blob/94f869998bb3fb76c074ac2aa60c3bfd131381f8/src/engine/physics.cpp#L1588
• https://github.com/sauerbraten/p1xbraten/blob/94f869998bb3fb76c074ac2aa60c3bfd131381f8/src/engine/physics.cpp#L1663

[sauerbraten]

the above code doesn't take into account spectating others.