feat: Initial document completion implementation

[Batalex]

Changes

• Initial completion implementation. There are most certainly some rough edges to polish, but I'd say this is a good starting point.
  https://github.com/user-attachments/assets/15ba1ac4-f534-4342-bf71-b34e68a3798b

Completion is context-aware; for instance, in the demo above, we see that the suggestion for "type" does not include "base" since we already have a base.

• While working on the completion feature, I realized we could improve on the hover lookup by iterating over the document nodes. Perfs are about the same, but the code is simpler and the UX better: we get the documentation by hovering not just on the key but on the value as well.
• Fixed a race condition where the completion endpoint would be triggered before the on_changed. Interestingly, the document version is set to None if we fetch it using a Path instead of a str