fast_i18n v2.0.0

Thanks to @DenchikBY.

• Now it's possible to set in and out directories for files.
• You can set the pattern by which to search for files.
• Generated keys can be switched to another case in generated classes.
• Removed dependency on devicelocale.
• Configs with baseLocale and maps moved from config.i18n.json to build.yaml
• Generators replaced with fields for keys with static values.
• Arguments now can be wrapped with braces like ${key}.
• Removed deprecated #map mode (deprecated in 1.5.0)

Example of new config in build.yaml:

targets:
  $default:
    builders:
      fast_i18n:i18nBuilder:
        options:
          base_locale: en
          input_directory: lib/i18n
          input_file_pattern: .i18n.json
          output_directory: lib/i18n
          output_file_pattern: .g.dart
          key_case: snake
          maps:
            - a
            - b
            - c.d