Convert newpipe/util/FilenameUtils.java to kotlin

[dustdfg]

What is it?

• Bugfix (user facing)
• Feature (user facing) ⚠️ Your PR must target the refactor branch
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

Description of the changes in your PR

• Converted to kotlin
• Simplified if-else statements to when
• Simplified handling of default value for selectedCharset
• Removed unnecessary (thanks to kotlin) string conversion to patterrn

APK testing

The APK can be found by going to the "Checks" tab below the title. On the left pane, click on "CI", scroll down to "artifacts" and click "app" to download the zip file which contains the debug APK of this PR. You can find more info and a video demonstration on this wiki page.

Due diligence

• I read the contribution guidelines.
• The proposed changes follow the AI policy.
• I tested the changes using an emulator or a physical device.