Port vstfxstore.h header.

I've copied the `version = VST_2_4_EXTENSIONS;` statement from `aeffect.d`. I'm not sure what the plan is, to only support 2.4+ (or 3.x+), or to hardcode these version statements into the file, or to set the version at the command-line to avoid hardcoding.

Btw, is `enum cMagic = "CcnK";` right or should this be a `ubyte[4]` instead? I guess comparisons will work either way.