initial FFmpeg support

First beta build of the audio processors system, now with basic FFmpeg support (short clip reading, live playback).

Migrate to v2

alpha5 to beta1 changes

Additions

• ClipName.FromPath method
• SecretLabNAudio.FFmpeg module
  • Extensions for the ShortClipCache to read clips with FFmpeg
  • Extensions for the AudioPlayer class to use FFmpeg
  • FFmpeg-based audio processors
    • AsyncBufferedFFmpegAudioProcessor
    • StreamBasedFFmpegAudioProcessor
    • SynchronousFFmpegAudioProcessor
  • FFmpeg installer
  • The module is included in the full SecretLabNAudio assembly

Fixes

• Fixed the embedded System.ValueTuple reference
• Fixed some docs

Changes (non-breaking)

• Updated ATTRIBUTIONS.md
• Moved some properties to Directory.Build.props
• Changed AsSpan() slicing to use the AsSpan(int, int) method

Full Changelog: v2.0.0-alpha5...v2.0.0-beta1