bin/langserver-jsts: fix script to work regardless of CDPATH
bin/langserver-jsts: fix script to work regardless of CDPATH

If CDPATH (a commonly used bash feature) is used, then this line to
determine the current directory does not work. The script would fail
with a Node error.

This fixes the script to work in that case.

sourcegraph/javascript-typescript-langserver,sourcegraph/javascript-typescript-langserver