feat(tracker): compile with midi support only when CGO is available

Also add the midi context to the VSTI, so VSTI can use MIDI if they
wish so.

cmd/midi_not_cgo.go

@@ -0,0 +1,12 @@
+//go:build !cgo
+
+package cmd
+
+import (
+	"github.com/vsariola/sointu/tracker"
+)
+
+func NewMidiContext(broker *tracker.Broker) tracker.MIDIContext {
+	// with no cgo, we cannot use MIDI, so return a null context
+	return tracker.NullMIDIContext{}
+}