Add GB National Insurance Numbers aka ID Numbers

In GB, we use [national insurance numbers](https://www.gov.uk/national-insurance/your-national-insurance-number) as our primary citizen identification numbers.

These are 9 digit strings that are typically 2 alpha characters followed by 6 numbers and finally a single alpha. There are few combinations are not permitted [as seen here](https://www.gov.uk/hmrc-internal-manuals/national-insurance-manual/nim39110).

The `valid` code provided will produce a subset of all legal NI numbers. A regex in the locale file to produce all combinations isn't necessary.

Author: stefanjcollier

lib/locales/en-GB.yml

@@ -85,6 +85,9 @@ en-GB:
         - Northern Ireland
       default_country_code:
         - GB
+    id_number:
+      valid: '/[A-CEGHJ-NOPR-TW-Z][ACEHJLMOPRSW][0-9]{6}[ABCD]/'
+      invalid: '/(BG|GB|NK|KN|TN|NT|ZZ)[0-9]{6}[E-Z]{1}/'
     internet:
       domain_suffix:
         - co.uk

test/test_en_gb_locale.rb

@@ -69,4 +69,18 @@ def test_en_gb_postcode_outcode_is_valid
     assert_match(/^[A-PR-UWYZ][A-HK-Y0-9]/, outcode)
     assert_match(/\w{1,2}\d{1,2}|\w\d[ABCDEFGHJKPSTUW]|\w\w\d[ABEHMNPRVWXY]/, outcode)
   end
+
+  def test_en_gb_id_number_valid_is_valid
+    id_code = Faker::IdNumber.valid
+
+    assert_equal(9, id_code.length)
+    assert_match(/^[A-CEGHJ-PR-TW-Z][A-CEGHJ-NPR-TW-Z][0-9]{6}[A-DFM]$/, id_code)
+  end
+
+  def test_en_gb_id_number_invalid_is_invalid
+    id_code = Faker::IdNumber.invalid
+
+    assert_equal(9, id_code.length)
+    assert_not_match(/^[A-CEGHJ-PR-TW-Z][A-CEGHJ-NPR-TW-Z][0-9]{6}[A-DFM]$/, id_code)
+  end
 end