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/MpcdecDecoderPlugin.cxx

@@ -268,15 +268,6 @@ mpcdec_scan_stream(InputStream &is, TagHandler &handler) noexcept
 
 static const char *const mpcdec_suffixes[] = { "mpc", nullptr };
 
-const struct DecoderPlugin mpcdec_decoder_plugin = {
-	"mpcdec",
-	nullptr,
-	nullptr,
-	mpcdec_decode,
-	nullptr,
-	nullptr,
-	mpcdec_scan_stream,
-	nullptr,
-	mpcdec_suffixes,
-	nullptr,
-};
+constexpr DecoderPlugin mpcdec_decoder_plugin =
+	DecoderPlugin("mpcdec", mpcdec_decode, mpcdec_scan_stream)
+	.WithSuffixes(mpcdec_suffixes);