fix: phone validation now only accepts 8 digit #s starting with 8 or 9

[karrui]

Problem

This PR adds an even strong mobile validation (only for Singapore numbers). SG phone number validation now only accepts 8 digit numbers starting with 8 or 9 using a regex match:

nationalNumber.match(/^[89][0-9]{7}$/g)

Closes #100

Solution

Bug Fixes:

• Increases mobile validation strength by also validating the start of the number in addition to the number length.

Before & After Screenshots

BEFORE:

AFTER:

[karrui]

Do not merge yet, realizes this stops home number from working. Will do the validation in the mobile phone check instead

EDIT: Fixed. Glad to see the tests worked to catch this ;)

[liangyuanruo]

lgtm