Rename Day01.kt to Day25.kt for keeping an template for the whole AoC

[MartinsCode]

I always forget to copy the solutions file to the next day before writing my solution. If the template would be Day25.kt, I'd be remebered every day, since filling in Day25.kt can only be done on the last day.

[sanderv]

I've created a live template in IntelliJ so that I can create an empty Day##.kt and then fill it with the template main() function including the correct name for the input files based on the Kotlin filename (template function fileNameWithoutExtension())