This commit adds support for "--eval-file" in "deno repl" subcommand.
This flag can be used to pass paths or URLs to files, that will be executed
on REPL startup. All files will be executed in the same context as the REPL
(ie. as "plain old scripts", not ES modules), sharing the global scope.
This feature allows to implement custom REPLs on top of Deno's REPL.
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.
