Add JS_SetLength API function

[andrjohns]

Partially addresses #443 by adding a new API function: JS_SetLength with corresponding internal function js_set_length64 to complement the existing JS_GetLength function

[andrjohns]

The netbsd CI failure seems unrelated/transient:

Run 'prepare' in VM
  Running prepare: /usr/sbin/pkg_add cmake gmake
  
  exec ssh: /usr/sbin/pkg_add cmake gmake
  /usr/bin/bash /home/runner/work/_actions/vmactions/netbsd-vm/v1/run.sh execSSHSH
  Config file: netbsd-10.0.conf
  pkg_add: no pkg found for 'cmake', sorry.
  pkg_add: 1 package addition failed