Bring extension update check in line with gh check

This commit is a bit of refactoring to bring the extension update checking logic up to par with what is done with `gh` including creation of state file per extension and listening to env vars for disabling version checking.

This work is not complete as it does not address necessary test changes.

pkg/cmd/extension/extension.go

@@ -41,7 +41,11 @@ type Extension struct {
 }
 
 func (e *Extension) Name() string {
-	return strings.TrimPrefix(filepath.Base(e.path), "gh-")
+	return strings.TrimPrefix(e.FullName(), "gh-")
+}
+
+func (e *Extension) FullName() string {
+	return filepath.Base(e.path)
 }
 
 func (e *Extension) Path() string {