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)

lib/encoder/os_other.go

@@ -0,0 +1,6 @@
+//+build !windows,!darwin
+
+package encoder
+
+// OS is the encoding used by the local backend for non windows platforms
+const OS = Base

lib/encoder/os_windows.go

@@ -0,0 +1,31 @@
+//+build windows
+
+package encoder
+
+// OS is the encoding used by the local backend for windows platforms
+//
+// List of replaced characters:
+//   < (less than)     -> '＜' // FULLWIDTH LESS-THAN SIGN
+//   > (greater than)  -> '＞' // FULLWIDTH GREATER-THAN SIGN
+//   : (colon)         -> '：' // FULLWIDTH COLON
+//   " (double quote)  -> '＂' // FULLWIDTH QUOTATION MARK
+//   \ (backslash)     -> '＼' // FULLWIDTH REVERSE SOLIDUS
+//   | (vertical line) -> '｜' // FULLWIDTH VERTICAL LINE
+//   ? (question mark) -> '？' // FULLWIDTH QUESTION MARK
+//   * (asterisk)      -> '＊' // FULLWIDTH ASTERISK
+//
+// Additionally names can't end with a period (.) or space ( ).
+// List of replaced characters:
+//   . (period)        -> '．' // FULLWIDTH FULL STOP
+//     (space)         -> '␠' // SYMBOL FOR SPACE
+//
+// Also encode invalid UTF-8 bytes as Go can't convert them to UTF-16.
+//
+// https://docs.microsoft.com/de-de/windows/desktop/FileIO/naming-a-file#naming-conventions
+const OS = (Base |
+	EncodeWin |
+	EncodeBackSlash |
+	EncodeCtl |
+	EncodeRightSpace |
+	EncodeRightPeriod |
+	EncodeInvalidUtf8)