Fix audio worklets in safari

[Danielku15]

Issues

Fixes #646

Proposed changes

Safari does not allow accessing the members of AudioWorkletGlobalScope through globalThis even though it provides it. It results in errors to do so. Therefore we need to ensure in alphaTab to not use globalThis to access the sampleRate and registerProcessor.

Using globalThis results in errors like:

TypeError: Can only call AudioWorkletGlobalScope.registerProcessor on instances of AudioWorkletGlobalScope

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• Existing builds tests pass
• New tests were added

Further details

• This is a breaking change
• This change will require update of the documentation/website