Add validator - "typeof"

[galhavivi]

Add a new validator to packages/form/src/validators.js file of "typeof".

Motivation
Fields can define a validation of their value type, to be verified when field value changes. For example to verify that the value of the field is "string".

Code

• args (and defaultArgs) should include:

• "type" - default undefined

• func: checks if : typeof value === args.type

• message: should have a matching error message depend on the args of
  'Invalid type. Expected type of "{args.type}"'

Tests

• should add unit tests to t all existing type that are listed here:
  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

Docs
should add documentation in Form -> Validators -> Build in validators:
https://yahoo.github.io/jafar/docs/validators.html#built-in-validators

CHANGELOG

• add a record to the CHANGELOG.md under the "Unreleased" section -> Form
  https://github.com/yahoo/jafar/blob/master/CHANGELOG.md