Kmp jvm support

[wilburx9]

This is the first in a line of PRs to support KMP.

Done

• Contrary to this suggestion, I opted for a single Timber and Tree class with platform-specific expected/actual functions. I did this to avoid code duplication and keep platform-specific code leaner. If this is an issue, I am happy to revert it and go with the linked suggestion.
• I migrated the log levels from android.util.Log to the Priority class with backward compatibility. However, this introduced a new class into the public API.
• [KMP] Add androidMain and commonMain, move current Timber to Android #558 inadvertently removed the tests from CI; this also fixes it.

Pending:

Here's some pending tasks that I can do here or another PR once there's a finalisation on the implementation:

1. Migrate the sample app to Compose with all supported targets
2. Support tests for the currently supported targets.
3. Timber.kt still references some java API; I plan to remove them in a future PR