Export CSSOM classes

[devingfx]

It can be usefull to be able to get and work with CSS classes, as it's already used in the library, why not make it accessible in root module and window like the rest of classes?

https://github.com/WebReflection/linkedom/blob/5527ee4aada82bd5cc07c739b87e372269922244/esm/html/style-element.js#L1

export * from 'cssom'