Display battery status on the extension icon and popup page, and support custom notifications.

Updates:
● Version 3.0.0 (2024/02/23): upgrade to Manifest V3 and requires Edge version >= 118.
● Version 2.0.2 (2023/02/08): support playing sound with notifications!
● Version 2.0.0 (2020/12/20): support custom notifications!

State of the extension icon:
● Green color: power level > 15%, Red color: power level <= 15%
● ⚡️flag: charging or fully charged, without ⚡️flag: not charged
● The battery icon itself reflects the overall power level
● Hover over the icon to display battery percentage

The popup page displays the battery detailed information. The options page let you set the extension icon and custom notifications.