fs/Path: rename GetSuffix() to GetExtension()

The "extension" is the name after the dot, but the "suffix" is the
string including the dot.

src/decoder/plugins/GmeDecoderPlugin.cxx

@@ -113,7 +113,7 @@ static AllocatedPath
 ReplaceSuffix(Path src,
 	      const PathTraitsFS::const_pointer new_suffix) noexcept
 {
-	const auto *old_suffix = src.GetSuffix();
+	const auto *old_suffix = src.GetExtension();
 	if (old_suffix == nullptr)
 		return nullptr;
 
@@ -322,7 +322,7 @@ gme_container_scan(Path path_fs)
 	if (num_songs < 2)
 		return list;
 
-	const auto *subtune_suffix = path_fs.GetSuffix();
+	const auto *subtune_suffix = path_fs.GetExtension();
 
 	TagBuilder tag_builder;