decoder/Plugin: simplify compile-time initialization

Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

src/decoder/plugins/ModplugDecoderPlugin.cxx

@@ -199,15 +199,7 @@ static const char *const mod_suffixes[] = {
 	nullptr
 };
 
-const struct DecoderPlugin modplug_decoder_plugin = {
-	"modplug",
-	modplug_decoder_init,
-	nullptr,
-	mod_decode,
-	nullptr,
-	nullptr,
-	modplug_scan_stream,
-	nullptr,
-	mod_suffixes,
-	nullptr,
-};
+constexpr DecoderPlugin modplug_decoder_plugin =
+	DecoderPlugin("modplug", mod_decode, modplug_scan_stream)
+	.WithInit(modplug_decoder_init)
+	.WithSuffixes(mod_suffixes);