zed_extension_api: Fork new version of extension API (#30611)

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

Release Notes:

- N/A

Cargo.lock

@@ -10037,7 +10037,7 @@ name = "perplexity"
 version = "0.1.0"
 dependencies = [
  "serde",
- "zed_extension_api 0.5.0",
+ "zed_extension_api 0.6.0",
 ]
 
 [[package]]
@@ -18695,7 +18695,7 @@ dependencies = [
 
 [[package]]
 name = "zed_extension_api"
-version = "0.5.0"
+version = "0.6.0"
 dependencies = [
  "serde",
  "serde_json",
@@ -18755,7 +18755,7 @@ dependencies = [
 name = "zed_test_extension"
 version = "0.1.0"
 dependencies = [
- "zed_extension_api 0.5.0",
+ "zed_extension_api 0.6.0",
 ]
 
 [[package]]