zed_extension_api: Fork new version of extension API (#44025)

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

We're jumping from v0.6.0 to v0.8.0 for the WIT because we released
v0.7.0 of the `zed_extension_api` without any WIT changes (it probably
should have been v0.6.1, instead).

Release Notes:

- N/A

crates/extension_api/src/extension_api.rs

@@ -334,7 +334,7 @@ mod wit {
 
     wit_bindgen::generate!({
         skip: ["init-extension"],
-        path: "./wit/since_v0.6.0",
+        path: "./wit/since_v0.8.0",
     });
 }