Static typing for AssetLoader

[alexeyinkin]

Currently EasyLocalization.assetLoader is dynamic.
It defaults to RootBundleAssetLoader that extends a local AssetLoader class.

easy_localization_loader has its own abstract AssetLoader, and its loaders extend it.

The following change will improve type safety:

1. Limit EasyLocalization.assetLoader to the local AssetLoader class.
2. Remove AssetLoader duplicate from easy_localization_loader.
3. In easy_localization_loader, depend on easy_localization and make all loaders extend its AssetLoader.

[bw-flagship]

Thanks for submitting this and explaining a solution! Maybe you want to submit a PR which implements that?

[alexeyinkin]

@bw-flagship done.

The plan is:

1. Merge Delete local AssetLoader class, update dependencies (#46) easy_localization_loader#50 and publish v2.0.0.
2. Merge Static type for EasyLocalization.assetLoader (#524) #598

Note that for both packages this is a breaking change, so the bump to 4.0.0 is required. You may want to hold back publishing until you do more breaking changes and cleanup that were coming.

[bw-flagship]

Why did you change the file names? It is not necessary for deduplicating the AssetLoader, isn't it?

[alexeyinkin]

I did not change them but sorted alphabetically. There is a lint for this: https://dart-lang.github.io/linter/lints/directives_ordering.html
Same goes for removing the library directive: https://dart.dev/effective-dart/style#dont-explicitly-name-libraries

I just did a quick scan of what I can easily fix. I can undo that if you want.

[bw-flagship]

I meant this breaking change:

- **BREAKING**: `JsonAssetLoader`, `XmlAssetLoader`, and `YamlAssetLoader` now use `_` instead of `-` when converting a locale to a file name.

Why is this change necessary?

[alexeyinkin]

These loaders were using localeToString function to convert from a Locale to the file name component:
https://github.com/aissat/easy_localization_loader/blob/0469058b11f65b92c4fdc7a2cbbf1900401df0d6/lib/src/yaml_asset_loader.dart#L12

This function was defined in both easy_localization_loaders and easy_localization, and is deprecated in the latter. The reason for deprecation is not mentioned, but my guess was that the author wanted to encourage more consistent naming of files, like en_US.yaml instead of en-US.yaml, since _ is the standard in Dart.

The straightforward way to keep the old behavior was to use the deprecated function from easy_localization, which I did not like. Following my guess of the purpose of the deprecation, I renamed the files.

When you asked, I discovered that Locale.toStringWithSeparator was introduced instead. So if we want to preserve the behavior, I can switch to using it.

Still the _ is more straightforward separator, and we can switch to defaulting to it at some point later.

[bw-flagship]

Thanks for explanation. I would prefer to preserve the old behavior to reduce breaking changes to a minimum :)

[alexeyinkin]

Done.