Add Deno.exitCode API

[lukeed]

Related to #23605

Implements the Deno.exitCode API to allow setting a would-be exit code without immediately terminating the process.

• Introduces a getter and setter for Deno.exitCode within runtime/js/30_os.js. The setter allows setting a would-be exit code that is stored in a private variable _exitCode. If a number is provided, it is parsed with parseInt and then passed to the op_set_exit_code Rust operation to update the process's exit code without exiting.
• Ensures that setting Deno.exitCode does not trigger an immediate process exit, allowing for asynchronous cleanup or other operations before the process terminates.

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[CLAassistant]

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[lukeed]

Closed in favor of #23609