feat: expose downloaded manifest statically + check if locale is supported according to manifest

[tim-alenus]

This PR makes the downloaded manifest (which happens during init) available under the static getter Crowdin.manifest.
This way once can check if a certain language is supported before calling await Crowdin.loadTranslations. Calling await Crowdin.loadTranslations results in an exception which is caught within the package. The function also does not return anything. With the manifest publicly exposed the app code can first check if it makes sense to call loadTranslations with a certain locale. One want to avoid to download the manifest again because every requests counts for the request count. Because this package already downloads the manifest I opted to make it publicly available.

[andrii-bodnar]

@tim-alenus thank you for the contribution!

Would it make more sense to check for language support inside the Crowdin.loadTranslations method?

[tim-alenus]

@andrii-bodnar that could be a useful addition as well. For this the manifest still needs to be saved somewhere though. There might be other use cases to expose the manifest, so having this publicly available still sounds like a good idea to me.

I will add a check in loadTranslations as well. In my opinion an exception should be thrown when a language is not supported. At the moment if you pass an unsupported language as parameter to loadTranslations the exception that occurs is caught and only causes a print in by the CrowdinLogger, but there is no way for the code that uses the package to known the language was not supported.

[andrii-bodnar]

Thank you for the contributions, @tim-alenus! I'm requesting a review from @FlutterOd.

[FlutterOd]

We mainly used logs because in case the translation from Crowdin is not available, the translation will just be fetched from the fallback file seamlessly.

Also, if we throw an exception with the loadTranslation method, it could cause problems for existing users who don't expect exceptions here and don't handle them.

So for the loadTranslations method, let's just check if the locale is supported and prevent requests if it isn't.

And you'll still have the checkManifestForLocale method if you need to handle the exception.

Suggested change

	if (manifest != null) {
	checkManifestForLocale(locale);
	}
	if (manifest != null) {
	try {
	checkManifestForLocale(locale);
	} catch (e) {
	CrowdinLogger.printLog(e.toString());
	_arb = null;
	return;
	}
	}