Modules: added jsVarNames() method.

Modules: added jsVarNames() method.

The method returns names of variables declared with js_var.  An optional
string argument filters returned names by prefix.

This lets JavaScript code discover a configuration-defined variable schema
without duplicating every variable name in the script.  For example:

    js_var $extract_name;
    js_var $extract_arguments_workspace;

    function handler(r) {
        r.jsVarNames("extract_").forEach(function(name) {
            r.variables[name] = extract(name.slice(8));
        });
    }

The same API is available as s.jsVarNames() in stream.  Variables created by
js_set or by the core modules are not returned.