Allow multiple prompters in deno_runtime crate (make PERMISSION_PROMPTER thread-local)

[carloslfu]

I'm running deno_runtime in multiple threads and trying to use different prompters to manage the permissions independently, but I hit a roadblock because PERMISSION_PROMPTER is static. This is an example of how I'm using deno_runtime: https://github.com/carloslfu/tauri-deno-example.

A solution could be using the thread_local! {} macro. I created this branch with the necessary changes and will submit a PR.