feat: Add Deno.exitCode API (#23609)

This commits adds the ability to set a would-be exit code 
for the Deno process without forcing an immediate exit, 
through the new `Deno.exitCode` API.

- **Implements `Deno.exitCode` getter and setter**: Adds support for
setting
and retrieving a would-be exit code via `Deno.exitCode`.
This allows for asynchronous cleanup before process termination 
without immediately exiting.
- **Ensures type safety**: The setter for `Deno.exitCode` validates that
the provided value is a number, throwing a TypeError if not, to ensure
that
only valid exit codes are set.

Closes to #23605

---------

Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>

runtime/js/99_main.js

@@ -674,6 +674,14 @@ ObjectDefineProperties(finalDenoNs, {
       return internals.future ? undefined : customInspect;
     },
   },
+  exitCode: {
+    get() {
+      return os.getExitCode();
+    },
+    set(value) {
+      os.setExitCode(value);
+    },
+  },
 });
 
 const {