Simplify the document preparation process

[freespirit]

As part of the setup, the user has to manually invoke the build_db step and provide the document. We could simplify this by lazily initializing the DB (parsing the document and embedding its pieces) at server/extension startup.

Check the zed_extensions_api for user-friendly functionality, e.g. progress reporting, during initialization of the extension.

[freespirit]

fn context_server_command() in lib.rs might be the right place to do this.
Alternatively, the slash command may require additional argument - the file. But this would make the first invocation slow and would change the logic a lot.