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

crates/extension_api/wit/since_v0.6.0/common.wit

@@ -0,0 +1,12 @@
+interface common {
+    /// A (half-open) range (`[start, end)`).
+    record range {
+        /// The start of the range (inclusive).
+        start: u32,
+        /// The end of the range (exclusive).
+        end: u32,
+    }
+
+    /// A list of environment variables.
+    type env-vars = list<tuple<string, string>>;
+}