Add ES6 module compilation

[Danielku15]

Issues

Fixes #666

Proposed changes

1. Adds detection on whether alphaTab is imported using ES6 modules and resolves the current script url accordingly
2. Adds ES6 module compilation output for usage in ES6 modules within the browser and respective handling on the workers.

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