feat(isMobilePhone): add Iceland phone validation and tests

[jonreiland]

A Regex validator has been added to /src/lib/isMobilePhone.js for Icelandic phone numbers, and added tests to /test/validators.test.js

I used this Wikipedia article as a reference, particularly the table included in the article which shows which numbers are valid for mobile phone numbers.

Checklist

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

[jonreiland]

@evanbechtol Thanks for the review!

[WikiRik]

Looking at the document linked here, the following ranges are also allowed;
385 000 000 - 385 999 999
388 000 000 - 388 999 999
389 000 000 - 389 999 999

Can you add those to the regex and tests?