feat: add string parser and validator

Author: emyann

src/validation/validators/string.validator.ts

@@ -0,0 +1,18 @@
+import { TypeValidator } from './interfaces.validator';
+import { isString } from '../../helpers';
+import { PropertyValidationError } from '../PropertyValidationError';
+
+export const string = Symbol('string');
+type StringValidator = TypeValidator<string, typeof string>;
+
+export const StringValidator: StringValidator = {
+  type: string,
+  validateAndParse: function(value) {
+    const result = value;
+    if (!isString(result)) {
+      throw new PropertyValidationError({ value, type: this.type });
+    } else {
+      return result;
+    }
+  }
+};