The PushMessageData interface mimics the Body interface, which was amended earlier this year with a new bytes() method, following the principle that APIs should generally vend byte buffers as Uint8Arrays.
(Re)align with the `Body` interface by providing the `bytes()` accessor on the `PushMessageData` interface as well. Today this can be done by obtaining the `arrayBuffer()` and then creating a Uint8 view based on it, however a principle was introduced that agreed we should make this the well lit path: https://github.com/w3ctag/design-principles/pull/480