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

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

[jonreiland]

@evanbechtol Thanks for the review!

[zispidd]

any updates?