Add normalizing notebook documents service

[msujew]

This is a continuation of #1660. Since the NotebookDocuments class exported by vscode-languageserver is quite specific in what it accepts as a TextDocuments instance (only instances of the class exported by vscode-languageserver), adopters of Langium, can no longer plug Langium's TextDocuments service in there.

However, the NotebookDocuments implementation of vscode-languageserver had the same URI encoding issues as TextDocuments anyway, and so we need to roll our own. This new NormalizingNotebookDocuments is now a default service of the shared LSP services and can be accessed by all Langium adopters.