[WIP] Fully convert project to Kotlin

[zacharee]

This PR converts all Java source files to Kotlin. The initial pass was done with Android Studio's automatic converter with manual changes made afterwards to fix errors and improve nullability indicators. Some code paths were also changed to better work with Kotlin.

I did some testing, but I'm sure there are bugs or unintended behavior changes I missed, so I'm marking this as work-in-progress and hoping to have others help test it.

The new codebase is also not API-compatible with the old version. Some of the changes can be tweaked so generated Java setter names match what they used to be, but Kotlin consumers will have to change to property access syntax, and the way Kotlin's generics work required adding more generic types to some classes.

[hannesa2]

As pull requests from a fork doesn't run screenshot compare tests, I'll make a pull request with given changes on upstream

[hannesa2]

Please see #414

[zacharee]

I fixed the unit tests and ported in the divide by 0 fix.

[zacharee]

Brought it back down to Java 17. There was some clashing with defaults that made me set it to 21, but 17 seems to work locally.

[hannesa2]

There is no progress in pull request from fork #413

[hannesa2]

@zacharee I don't want to eliminate you motivation to contribute here, but I guess, your probably AI generated, huge change neither run the Espresso tests properly, nor builds to code properly.
As I do this Kotlin conversion slowly human-made, step by step, your pull request run into a lot of conflicts.

If you want to continue here, please start again from recent master

[zacharee]

This wasn't AI generated, but I lost the time to maintain it.

[hannesa2]

@zacharee
I want to warmly invite you, to continue here, based on recent master.
Currently I fight with class ChartData ...

[hannesa2]

This https://github.com/AppDevNext/AndroidChart/releases/tag/4.0-beta3 is a Kotlin only version.
But there are some compromises:

• you have to use setData() in Kotlin. I try to fix with Data as var #614
• There is a new package name. I keep old Chart classes, but this only points you to change the used package names, because you have to change packages anyway, when you use some sub-classes