Use AudioWorklets for synthesizer

[Danielku15]

AudioWorklets provide a nice way of synthesizing audio in the a background worker instead of the UI thread. It would be worth a test synthesizing the audio via worklet to avoid hanging audio in the UI.

Various things will need to be considered:

• How can we send custom messages from the worklet to the UI for synth events.
• Check if the worklet would only syntheiszes one buffer and buffering is done via nodes, or if the worklet would spawn another worker for synthesizing and do a custom buffering like the output today.
• AudioWorklets are only available in secure contexts and hence the old processor node still needs to be available in parallel.

[Danielku15]

AudioWorklets are now used when available by the browser and security context. the old-fashioned script processor is still in place as fallback scenario.