feat(isMobilePhone): add El Salvador es-SV locale

* Solve issue with wrong validation on El Salvador mobile phones
* update tests accordingly

in order to define the right regEx, the following site was
consulted.

https://www.siget.gob.sv/guia-de-servicios/consulta-el-plan-de-numeracion/numeros-moviles/

src/lib/isMobilePhone.js

@@ -70,6 +70,7 @@ const phones = {
   'es-MX': /^(\+?52)?(1|01)?\d{10,11}$/,
   'es-PA': /^(\+?507)\d{7,8}$/,
   'es-PY': /^(\+?595|0)9[9876]\d{7}$/,
+  'es-SV': /^(\+?503)?[67]\d{7}$/,
   'es-UY': /^(\+598|0)9[1-9][\d]{6}$/,
   'es-VE': /^(\+?58)?(2|4)\d{9}$/,
   'et-EE': /^(\+?372)?\s?(5|8[1-4])\s?([0-9]\s?){6,7}$/,

test/validators.js

@@ -7459,6 +7459,30 @@ describe('Validators', () => {
           '+591993546843',
         ],
       },
+      {
+        locale: 'es-SV',
+        valid: [
+          '62136634',
+          '50361366631',
+          '+50361366634',
+          '+50361367217',
+          '+50361367460',
+          '+50371367632',
+          '+50371367767',
+          '+50371368314',
+        ],
+        invalid: [
+          '+5032136663',
+          '21346663',
+          '+50321366663',
+          '12345',
+          'El salvador',
+          'this should fail',
+          '+5032222',
+          '+503 1111 1111',
+          '00 +503 1234 5678',
+        ],
+      },
       {
         locale: 'es-UY',
         valid: [