add difficulty tag for problems

[Raysuner]

🚀 Feature Proposal

could you add the difficulty tag for each problem ,for example hard, mid,easy

Motivation

I want to solve problem from easy to hard on the same type of problem. if so, This will be more convenient than before

Contribution

Please describe the solution if you'd like to submit a PR.

[Vigilans]

Maybe it could be eased by adding information tooltip to problems.

[Vigilans]

Or something like file explorer's green(created), yellow(modified), red(errors) would be fine?

[Raysuner]

I think it can refer to leetcode offical website