Allow for localized snippets

[salmenf]

Package snippets should be localized. To avoid an overcrowded package.json, localization can be implemented such that each localized snippet is in a separate file ./snippets/my-snippet.de.html, and the syntax in the package.json is changed to "./snippets/snippet-name.*": "./snippets/snippet-name.*". WebWriter then only shows the snippets for the document locale (falling back to English if no other is available).