Leetcode Difficulty Rating per boenshao

Replace Leetcode problem's difficulty with a more precise contest rating.

Support both "leetcode.com" and "leetcode.cn".

The green/yellow/red text color is preserved, so you can still tell the official difficulty.

Problems in 1st-62nd weekly contests and problems that did not come from contests don’t have rating data and "N/A" is shown by default. To show the original rating, click the extension icon on the top right of the browser and disable "Show N/A if no rating is available".

For more details and source code, please visit:
- https://github.com/boenshao/leetcode-difficulty-rating-extension