Update ZIO and enable Native multi-threading

[kyri-petrou]

Main changes:

• Updates ZIO to 2.1.8
• Enables multi-threading for Scala Native
• Uses the releaseFast mode for MacOS (see https://github.com/zio/zio/releases/tag/v2.1.8)
• Added the .sbtopts file with sensible config. This is needed because the default memory settings (-Xmx1G) is not enough to compile Scala Native

I also did some minor adjustments to CI to speed things up:

• Set -Xmx and -Xms to 6G in CI
• use testNative and testJS so that only modules that will run tests are linked