refactor!: rename SynthService to Synther and related types

The -er suffix is more idiomatic for single method interfaces, and
the interface is not doing much more than converting the patch to a
synth. Names were updated throughout the project to reflect this
change. In particular, the "Service" in SynthService was not telling
anything helpful.

cmd/main_synther_not_native.go

@@ -0,0 +1,7 @@
+//go:build !native
+
+package cmd
+
+import "github.com/vsariola/sointu/vm"
+
+var MainSynther = vm.GoSynther{}