153. uglify CSS class names

If you use css-loader in your webpack project, localIdentName could be used to transform class names, like below:

localIdentName: "[path][name]__[local]--[hash:base64:5]",

Or you can create your own function to generate class names by setting getLocalIdent.

Now please create a function to generate unique class names following rules below.

1. only use alphabets: a to z , A to Z
2. return one unique class name each time function is called
3. the class name sequence should first be in order of length, then in Alphabetical order(lowercase in front).
4. should provide a function to reset the sequence

getUniqueClassName()
// 'a'

getUniqueClassName()
// 'b'

getUniqueClassName()
// 'c'

// skip cases till 'Y'

getUniqueClassName()
// 'Z'

getUniqueClassName()
// 'aa'

getUniqueClassName()
// 'ab'

getUniqueClassName()
// 'ac'

// skip more cases

getUniqueClassName()
// 'ZZ'

getUniqueClassName()
// 'aaa'

getUniqueClassName()
// 'aab'

getUniqueClassName()
// 'aac'

getUniqueClassName.reset()

getUniqueClassName()
// 'a'