feat: support fully independent isolates

[edusperoni]

Currently adds supports to nested workers. This is the first step in supporting multiple main thread isolates.

• Make worker isolates independent from threading and from which isolate they should work from
• Allow passing of custom runloops to the runtime, so the runtime itself can be initialized in one runloop, but work in another
• Allow creating multiple runtimes in the same runloop/thread
• manage disposing of the main runloop