[2.0.0-alpha2] Formatters: Remove Twig-based slot rewrite feature

Problem/Motivation

In Alpha 1, ui_patterns_field_formatters was shipped with a mechanism inspired form Views, which allow the use of Twig to rewrite a slot output.

There are a few issues about that:

• It is an addition which was not in the UI Patterns 1.x scope, so it doesn't belong to 2.0 which is a rewrite of the current scope.
• It is for power users, and may be confusing for the general audience.
• It add complexity to an already heavy codebase
• It is not specific to Field Formatters, so may belong to a shared API.
• Other similar power users features may be implemented this way (remove markup, tokens...)

Proposed resolution

In UI Patterns 2.0:

• Add a way of extending Form Builder and/or Source plugins forms. Check with Christian because he will need this for UI Patterns Settings 3.x too
• Move this mechanism to a separate module (outside of the codebase), using this extension capacity

In UI Patterns 2.1 or superior:

• If it was not added to UI Patterns Settings 3.x, let's talk about adding a ui_patterns_advanced sub moduels with this kind of stuff