Restore package license metadata

[Errordog2]

Closes #232.

This restores the package license metadata for xendit-node.

Historical context:

• Open-source License #37 requested adding an open-source license, "probably MIT".
• LICENSE & miscellaneous docs updates #41 was merged with an MIT LICENSE file for this package.

The current default branch and published xendit-node@7.0.0 package no longer expose a machine-readable license. This PR restores:

• license: "MIT" in package.json, so the generated/published package declares its license
• the top-level MIT LICENSE file text from the previously merged license PR

Validation:

• package.json parses successfully after the change
• checked that package.json now contains license: "MIT"
• change is limited to license metadata/documentation; no runtime code, dependencies, exports, or generated API files changed