Fixes "op_set_exit_code" by sharing a single "Arc" between
all workers (via "op state") instead of having a "global" value stored in
"deno_runtime" crate. As a consequence setting an exit code is always
scoped to a tree of workers, instead of being overridable if there are
multiple worker tree (like in "deno test --jobs" subcommand).
Refactored "cli/main.rs" functions to return "Result<i32, AnyError>" instead
of "Result<(), AnyError>" so they can return exit code.
