Introduce the ability to talk to GPT-4 within buffers on preview and dev (#2524)

This adds basic ability to interact with OpenAI inside a buffer.

Release Notes:

* If`OPENAI_API_KEY` is defined in your environment, you can now run the
`ai: assist` command to pass the text of your current buffer to GPT-4.
If you're editing a file with a `.zmd` extension, you can also invoke
the model with `cmd-enter.` (preview-only)

crates/workspace/Cargo.toml

@@ -62,5 +62,5 @@ settings = { path = "../settings", features = ["test-support"] }
 fs = { path = "../fs", features = ["test-support"] }
 db = { path = "../db", features = ["test-support"] }
 
-indoc = "1.0.4"
+indoc.workspace = true
 env_logger.workspace = true