Make ISynthOutput implementations public again.

[Danielku15]

It seems we accidentally in a previous release have hidden classes like AlphaSynthWebAudioOutput, they should be public again for low level api usage. Likely the guide also needs update as the API changed in 1.3 with audioworklets.

Discussed in #895

^{Originally posted by TheLucifurry June 8, 2022}
AlphaSynthWebAudioOutput
I just can't find how to use it and where it's from in this manual)
https://www.alphatab.net/docs/guides/lowlevel-apis#playing-files-via-alphasynth