Revert "Make SinkContext a struct"

It's better as an interface so users don't have to call the `Sink`
method on the struct during Make.

- Move SinkContext into context.go

Author: ammario

context.go

@@ -4,15 +4,21 @@ import "context"
 
 type loggerCtxKey = struct{}
 
-// SinkContext is used by slog.Make to compose many loggers together.
-type SinkContext struct {
+type sinkContext struct {
+	context.Context
+	Sink
+}
+
+// SinkContext is a context that implements Sink.
+// It may be returned by log creators to allow for composition.
+type SinkContext interface {
 	Sink
 	context.Context
 }
 
 func contextWithLogger(ctx context.Context, l logger) SinkContext {
 	ctx = context.WithValue(ctx, loggerCtxKey{}, l)
-	return SinkContext{
+	return &sinkContext{
 		Context: ctx,
 		Sink:    l,
 	}