Adapt non-android specific changes from android branch

[Danielku15]

Issues

Relates to #532 and #612

Proposed changes

Adapts some first changes required for the android integration but are not specific to it:

• Add explicit overrides to TS codebase (code style)
• Optimized Kotlin/C# generation to only generate virtual/open only when needed (code style and kotlin compilation perf)
• Add strict JSON parsing for score (general runtime performance)
• Improvement map type generation (Kotlin runtime performance)
• Adapted some model properties to hold null when unused to avoid allocation of empty arrays (general memory performance)
• Reworked linking of notes after serialization (cleaner data model)
• Split code for serializer emitter to multiple files and adapted a string->ts.Node conversion approach where reasonable (code style)

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