zed_extension_api: Fork new version of extension API (#17795)

This PR forks a new version of the `zed_extension_api` in preparation
for new changes.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

extensions/test-extension/Cargo.toml

@@ -13,4 +13,4 @@ path = "src/test_extension.rs"
 crate-type = ["cdylib"]
 
 [dependencies]
-zed_extension_api = "0.1.0"
+zed_extension_api = { path = "../../crates/extension_api" }