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)

Cargo.lock

@@ -95,6 +95,22 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "ai"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "assets",
+ "collections",
+ "editor",
+ "futures 0.3.28",
+ "gpui",
+ "isahc",
+ "serde",
+ "serde_json",
+ "util",
+]
+
 [[package]]
 name = "alacritty_config"
 version = "0.1.1-dev"
@@ -8762,6 +8778,7 @@ name = "zed"
 version = "0.89.0"
 dependencies = [
  "activity_indicator",
+ "ai",
  "anyhow",
  "assets",
  "async-compression",