VSTSID - Commodore 64 SID synthesizer plugin

As requested by many WebSID users: a VST plugin version of the previously web-only "Commodore 64 synthesizer".

The synthesis (and arpeggiation) code has been ported from the JavaScript project into C++. The plugin has been built on top of Steinbergs VST SDK and can thus be built for Windows, MacOS and Linux platforms.

The plugin can run in any VST3.0 compliant host/DAW, which include Bitwig Studio, Reaper, Plogue Bidule, etc. Instructions are provided to build as a VST2.4 plugin as well as building for both 32-bit and 64-bit architectures.