local: refactor default os encoding out from local backend into shared encoder lib

lib/encoder/os_darwin.go

@@ -0,0 +1,9 @@
+//+build darwin
+
+package encoder
+
+// OS is the encoding used by the local backend for macOS
+//
+// macOS can't store invalid UTF-8, it converts them into %XX encoding
+const OS = (Base |
+	EncodeInvalidUtf8)