Adding JavaScript syntax support for JavaScriptBuild (.jsb) files

jsb is a meta build system [1] which can generate actual build files for GNU make, ninja, visual studio etc.

These files are pure javascript files. Just to differentiate them from rest of the javascript files, these are marked as .jsb file.

[1] https://github.com/vivekgalatage/jsb/blob/master/third-party/libxml/libxml.jsb