refactor: focus around Render/Compact/Pretty/NewWith functions

This is yet another drastic refactor of public API and concepts.
Hopefully the last one, as I'm now fairly happy with things.

interfaces.go

@@ -0,0 +1,41 @@
+package render
+
+import "io"
+
+// Handler interface is for single format renderers, which can only render a
+// single format. It is the basis of the multi-format support offerred by the
+// render package.
+type Handler interface {
+	// Render writes v into w in the format that the Handler supports.
+	//
+	// If v does not implement a required interface, or otherwise cannot be
+	// rendered to the format in question, then a ErrCannotRender error must be
+	// returned. Any other errors should be returned as is.
+	Render(w io.Writer, v any) error
+}
+
+// PrettyHandler interface is a optional interface that can be implemented by
+// Handler implementations to render a value in a pretty way. This is
+// useful for formats that support pretty printing, like in the case of JSON and
+// XML.
+type PrettyHandler interface {
+	// RenderPretty writes v into w in the format that the Handler supports,
+	// using a pretty variant of the format. The exact definition of "pretty" is
+	// up to the handler. Typically this would be mean adding line breaks and
+	// indentation, like in the case of JSON and XML.
+	//
+	// If v does not implement a required interface, or otherwise cannot be
+	// rendered to the format in question, then a ErrCannotRender error must be
+	// returned. Any other errors should be returned as is.
+	RenderPretty(w io.Writer, v any) error
+}
+
+// FormatsHandler is an optional interface that can be implemented by Handler
+// implementations to return a list of formats that the handler supports. This
+// is used by the New function to allow format aliases like "yml" for "yaml".
+type FormatsHandler interface {
+	// Formats returns a list of strings which all target the same format. In
+	// most cases this would just be a single value, but multiple values are
+	// supported for the sake of aliases, like "yaml" and "yml".
+	Formats() []string
+}