JavaScript:实现检查给定的字符串是否是扁平(全部小写)的算法
// checkFlatCase method checks if the given string is in flatcase or not. Flatcase is a convention
// where all letters are in lowercase, and there are no spaces between words.
// thisvariable is an example of flatcase. In camelCase it would be thisVariable, snake_case this_variable and so on.
// Problem Source & Explanation: https://en.wikipedia.org/wiki/Naming_convention_(programming)
/**
* checkFlatCase method returns true if the string in flatcase, else return the false.
* @param {string} varname the name of the variable to check.
* @return