Create a custom launcher core

[kanpov]

The previous plan was to use GJMLC as a launcher core in this app, but it has some issues which have driven me to the conclusion of making our own launcher:

• It is no longer supported. The repository hasn't been receiving any sort of updates since 2016
• It is not on Maven Central. This makes depending on it hard (previously we just inlined the entire code into a new package)
• It is outdated. Some parsing methods do not apply to the newest Mojang launcher formats, so a rework would be necessary anyway
• It has bad code quality. The core would need some cleaning up before it's code becomes good
• It is written in Java. As you may know, this project is fully in Kotlin, and adopting a Java library (especially a core component for the app to function correctly) is a bad idea, even though the interoperability works.
• Most importantly, it only supports legacy launcher formats, which means that a huge rework for that would also be needed if we had decided to stick with GJMLC.

I cannot give any estimates on when this will be ready, and there will likely be conflicts with design-phase-zero branch.

[kanpov]

This PR is almost done.

Fabric, Forge, Quilt, profiles etc will come in other PRs.

All that's left to do is fix the launching system and add support for authentication.

[kanpov]

Minecraft launches now.

Auth, assets and asset indexes are the only things left to implement.

[kanpov]

This PR is almost ready for merge.

Waiting a bit for the CI to finish and merging it.