extention_directory.js

var support_dir = brackets.app.getApplicationSupportDirectory(),
    precursor_path,
    precursor_file = "package.json";   // where this represents some config file.

support_dir += "/extensions/user/zeffii.precursor/";
precursor_path = support_dir + precursor_file;

var prototypes;

$.getJSON(precursor_path, function (data) {
    prototypes = data;
});