feat(isMobilePhone): add Iceland (is-IS) mobile phone validation

[Sagar141005]

This PR adds support for validating Icelandic mobile phone numbers (is-IS) in the isMobilePhone validator.
Specifically:

• Added a regex for Iceland in src/lib/isMobilePhone.js: 'is-IS': /^(\+?354|0)[5-8]\d{6}$/
• Added tests in test/validators.test.js covering valid and invalid Icelandic phone numbers.
• Updated README.md to include is-IS as a supported locale.

References:

• Icelandic phone number formats
• Consistent pattern with existing isMobilePhone locales.

Checklist

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

Issue: #1761

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (784e52a) to head (41c2bdb).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2654   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2572      2572           
  Branches       651       651           
=========================================
  Hits          2572      2572           

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