[IndexedDB] Adhere better to the specification for idb object store related operations (#37682)

Many object store related operations require the transaction to be
checked: to ensure it is still active, and, if the operation is a write,
that the transaction is not read-only. I've added the
`check_transaction` method to perform these checks.

Additionally `Clear` was still half-implemented, so I went ahead and
implemented that.

---------

Signed-off-by: Ashwin Naren <arihant2math@gmail.com>

components/net/indexeddb/engines/heed.rs

@@ -262,6 +262,16 @@ impl KvsEngine for HeedEngine {
                                 .expect("Could not get store");
                             // FIXME:(arihant2math) Return count with sender
                         },
+                        AsyncOperation::ReadWrite(AsyncReadWriteOperation::Clear) => {
+                            let stores = stores
+                                .write()
+                                .expect("Could not acquire write lock on stores");
+                            let store = stores
+                                .get(&request.store_name)
+                                .expect("Could not get store");
+                            // FIXME:(arihant2math) Error handling
+                            let _ = store.inner.clear(&mut wtxn);
+                        },
                     }
                 }