Add support for copy and paste in WSL

- clip.exe is used to copy to the Windows clipboard
- There's no binary for pasting from the Windows clipboard so
  `Get-Clipboard` from powershell is used as a workaround. The
  superflous carriage return is stripped from the output.

share/functions/fish_clipboard_copy.fish

@@ -12,5 +12,7 @@ function fish_clipboard_copy
         printf '%s' $cmdline | xsel --clipboard 2>/dev/null
     else if type -q xclip
         printf '%s' $cmdline | xclip -selection clipboard 2>/dev/null
+    else if type -q clip.exe
+        printf '%s' $cmdline | clip.exe
     end
 end

share/functions/fish_clipboard_paste.fish

@@ -8,6 +8,8 @@ function fish_clipboard_paste
         set data (xsel --clipboard 2>/dev/null)
     else if type -q xclip
         set data (xclip -selection clipboard -o 2>/dev/null)
+    else if type -q powershell.exe
+        set data (powershell.exe Get-Clipboard | string trim -r -c \r)
     end
 
     # Issue 6254: Handle zero-length clipboard content