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

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

src/input/plugins/ArchiveInputPlugin.cxx

@@ -49,7 +49,7 @@ OpenArchiveInputStream(Path path, Mutex &mutex)
 		return nullptr;
 	}
 
-	const char *suffix = l.archive.GetSuffix();
+	const char *suffix = l.archive.GetExtension();
 	if (suffix == nullptr)
 		return nullptr;