LibWeb: Align AudioContext constructors with current specification steps

[tcl3]

This PR updates the spec steps for the AudioContext and OfflineAudioContext constructors.

While working on this I found a few minor inconsistencies, which I believe are (minor) spec issues:

• The AudioContext spec steps don't mention validating the given sample rate. The /webaudio/the-audio-api/the-audiocontext-interface/audiocontextoptions.html test expects this parameter to be validated.
• The spec only mentions validating OfflineAudioContext parameters for the constructor that takes individual arguments - not the constructor that takes an OfflineAudioContextOptions object.
• The AudioContext steps don't include a step to "Determine the [[render quantum size]]". We don't currently implement this, so it's not currently a problem for us.
• The OfflineAudioContext constructor steps should probably end with a Return c step. This would match the AudioContext constructor steps.