Use a more sensible format for Yaml translation pattern definition.

There have been a few issues with the current format for Yaml translation patterns since it was proposed and committed. So, I have decided to propose a more sensible format for defining these patterns.

There are two main issues with the current format:

1. Using special symbols for Yaml (e.g. * and %) in the format. Potx is currently only using Yaml's Lexer, so it doesn't cause an issue now. But Potx will eventually be ported to drupal 8, and Yaml's Parser will trip over these symbols. This had already caused an issue when mixing Potx with drush 8.x in #2371771: Default translation pattern keys considered as references by the Yaml parser.
2. There is no point of extension in the current format. This forced a hack to define a special value ('%top-level-key') to fix #2311935: Add support for .permissions.yml files, and is going to cause issue for defining a way to import/inherit translation patterns between modules.

Here is a sample of the new format, based on the core translatable Yaml files:

translation_patterns:
  - matches: '*.info.yml'
    translatable_keys:
      - name
      - description
      - package
      - regions
  - matches: '*.routing.yml'
    translatable_keys:
      - key: _title
        context: _title_context
  - matches: '*.links.*.yml'
    translatable_keys:
      - key: title
        context: title_context
  - matches: '*.links.menu.yml'
    translatable_keys:
      - description
  - matches: '*.breakpoints.yml'
    translatable_keys:
      - label
      - group
  - matches: '*.permissions.yml'
    top_level_translatables: true
    translatable_keys:
      - title
      - description
  - matches: 'config/schema/*.yml'
    translatable_keys:
      - label


inherits:
  - module_x
  - module_y

The format is more verbose now. The list of patterns have been moved into the "translation_patterns:" key, which allows extending the format at top-level, like the current "inherits:" key. The list of translatable keys has also moved into "translatable_keys:", which adds another point of extension, that allowed replacing the mentioned '%top-level-keys' hack with a more appropriate fix, using the 'top_level_translatables' key in the proposed format.

The amount of verbosity, and the namings can be discussed here, but I think this format is a lot easier to maintain and extend in the long run.