fix(watch): keep watcher alive when script calls Deno.exit()
When a script run with `deno run --watch` (or `deno serve --watch`) called
`Deno.exit()`, the op went straight to `std::process::exit()`, killing the
whole watcher process instead of just ending the current run.
Mirror the approach the test runner already uses for `Deno.exit()` inside a
test: when a watcher is active, `run_for_watcher` installs the isolate handle
in `OpState` so `op_exit` terminates the current V8 isolate instead of the
process. The watcher then clears the termination flag and treats it as a
normal end of run, restarting on the next file change.
Closes #7590
