Add a trailing slash to a string, if it doesn't already have one.
If using Node.js, it will replace the trailing slash based on the value of os.platform
(i.e. if windows, '\\' will be replaced, '/' otherwise).
@param {String} s The string to operate on.
@returns {String} The string without a trailing slash.
@function
function withTrailingSlash (s) {
                    if(!IS_BROWSER && HAS_OS && require('os').platform === 'win32') return libs.string.withoutTrailingSlash(s) + '\\';
                    return libs.string.withoutTrailingSlash(s) + '/';
                }