feat(forms/validators): pattern validator

Adding static pattern validation method to Validators

Adding a directive for the pattern validator

Applying clang-format rules to modified files

Updating public api spec for new pattern validator

Adding pattern validator to public api guard tool

For #5411

Closes #5561

tools/public_api_guard/public_api_spec.ts

@@ -772,6 +772,9 @@ const COMMON = [
   'NgSwitchWhen.constructor(viewContainer:ViewContainerRef, templateRef:TemplateRef, ngSwitch:NgSwitch)',
   'NgSwitchWhen.ngSwitchWhen=(value:any)',
   'NumberPipe',
+  'PatternValidator',
+  'PatternValidator.constructor(pattern:string)',
+  'PatternValidator.validate(c:Control):{[key:string]:any}',
   'PercentPipe',
   'PercentPipe.transform(value:any, args:any[]):string',
   'RequiredValidator',
@@ -795,6 +798,7 @@ const COMMON = [
   'Validators.maxLength(maxLength:number):Function',
   'Validators.minLength(minLength:number):Function',
   'Validators.nullValidator(c:any):{[key:string]:boolean}',
+  'Validators.pattern(pattern:string):Function',
   'Validators.required(control:Control):{[key:string]:boolean}',
   'RadioButtonState',
   'RadioButtonState.constructor(checked:boolean, value:string)',