Support per-document read lock

[msujew]

This PR attempts to fix a similar problem as our implementation already does inside of Langium. Some services require only specific document states to be available. For example, a document symbol request can be immediately resolved, once the document has been succesfully parsed - without waiting for the build to finish, or even references to be linked.

This change adds two main features for this:

1. Adds a ReadAt method to the Lock interface, which allows to read documents at specific states.
2. Adds a DocumentStateRequirements interface, that LSP services can (optionally) implement to start reading at a specific workspace/document state. Any service that does not implement this interface falls back to Read.

Removes the downgrade functionality from the lock, which has been replaced with ReadAt. Write and Read are now fully exclusive.

Related to eclipse-langium/langium#2214.