chore: set root crate license to MIT

[Boshen]

The root oxc-node crate has no license field in Cargo.toml, so cargo deny (run by the Security Analysis workflow) now fails it as unlicensed:

warning[no-license-field]: license expression was not specified in manifest for crate 'oxc-node = 0.0.0'
error[unlicensed]: oxc-node = 0.0.0 is unlicensed
...
advisories ok, bans ok, licenses FAILED, sources ok

This started failing across branches today (the action installs cargo-deny fresh, and it now errors on the pre-existing missing field). Declaring the license fixes it repo-wide.

MIT matches the license already published for @oxc-node/core and @oxc-node/cli.

[package]
name = "oxc-node"
version = "0.0.0"
authors = ["LongYinan <github@lyn.one>"]
edition = "2024"
license = "MIT"
publish = false

Only Cargo.toml changes — Cargo.lock is unaffected. cargo metadata confirms the crate now resolves with "license":"MIT".