feat(plugins): expose document path to plugins via HostApi

OpenCADStudio/plugins#901 (comment)

- Add HostApi::document_path(tab_id) returning Option<PathBuf>.
- Add PluginRequest::DocumentPath and PluginResponse::DocumentPath for
  out-of-process plugins.
- Implement the request in the IPC server and in both V3/V4 plugin-side
  HostApi proxies.
- Implement HostSession::document_path by looking up the tab by id and
  returning its current_path.

crates/ocs_plugin_api/src/ipc/server.rs

@@ -74,5 +74,8 @@ pub fn handle_plugin_request(
             PluginResponse::Ok
         }
         GetTabId => PluginResponse::TabId(host.tab_id()),
+        DocumentPath { tab_id } => {
+            PluginResponse::DocumentPath(host.document_path(tab_id).map(|p| p.to_string_lossy().to_string()))
+        }
     }
 }