Add password-field base class using new mixins

[web-padawan]

Motivation

This is a pre-requisite for adding slotted <input> support to vaadin-password-field.

Depends on #2189 (needs text-field base class)
Part of #84

Proposed solution

1. Create a new package @vaadin/password-field for a base class
2. Add a class without defining a custom element - prototype
3. Copy vaadin-password-field test suite, excluding tests covered by @vaadin/field-base mixins tests.

Related a11y issues

Reveal button not keyboard accessible (#120).

[knoobie]

I have two notes about this from my test with our screen reader user in the past:

• the aria-label of the button has to be customizable, otherwise it's another a11y/i18n problem
• is it possible that the reveal button could work different depending how it was triggered?
  • Triggered by mouse: no change needed
  • Triggered by keyboard: the reveal/un-reveal mechanism should not be triggered by focus change, instead it should only be done manually by another click on the reveal button (Reasoning: screen reader have to change focus from the button to the input field, to read it.. this changes the type to password again and the screen reader can't read the value)

Edit: another idea; instead of changing the behavior depending on the trigger.. could the field value be announced like we did with the a11y announcer in vaadin avatar group?