FEAT: Updated the readme and added tests for validator of Marshall Islands country

[poswalsameer]

This PR solves my previous issue of not updating the readme and I have added tests too for validation of the mobile number

Fixes #1761

Checklist

• [Yes] PR contains only changes related; no stray files, etc.
• [Yes] README updated (where applicable)
• [Yes] Tests written (where applicable)
• [Yes] References provided in PR (where applicable)

[rubiin]

@poswalsameer could you fix the failed tests. Also there are two PRs for the same thing https://github.com/validatorjs/validator.js/pull/2365