AtCoder page mover using arrow keys

# AtCoder page mover using arrow keys
This is a Chrome extension for moving between AtCoder pages using arrow keys.

## How to use
1. Open an AtCoder problem page (e.g. `https://atcoder.jp/contests/abc200/tasks/abc200_a`) in your browser.
2. Use the arrow keys to move between pages.
- `→`: Move to the next problem page.
- e.g. from `... /abc200_a` to `... /abc200_b`
- `←`: Move to the previous problem page.
- e.g. from `... /abc200_b` to `... /abc200_a`
3. You can also move between pages by pressing a key of a task
- e.g. from `.../abc200_a` to `.../abc200_c` by pressing the key `c`

## Note
- If the next or previous page does not exist, nothing is done.