command: allow targeting outputs by name

This extends focus-output and send-to-output to allow targeting
outputs by name instead of relative position.

river/command.zig

@@ -99,6 +99,7 @@ pub const Error = error{
     InvalidCharacter,
     InvalidDirection,
     InvalidPhysicalDirection,
+    InvalidOutputIndicator,
     InvalidOrientation,
     InvalidRgba,
     InvalidValue,
@@ -141,6 +142,7 @@ pub fn errToMsg(err: Error) [:0]const u8 {
         Error.InvalidCharacter => "invalid character in argument",
         Error.InvalidDirection => "invalid direction. Must be 'next' or 'previous'",
         Error.InvalidPhysicalDirection => "invalid direction. Must be 'up', 'down', 'left' or 'right'",
+        Error.InvalidOutputIndicator => "invalid indicator for an output. Must be 'next', 'previous', 'up', 'down', 'left', 'right' or a valid output name",
         Error.InvalidOrientation => "invalid orientation. Must be 'horizontal', or 'vertical'",
         Error.InvalidRgba => "invalid color format, must be hexadecimal 0xRRGGBB or 0xRRGGBBAA",
         Error.InvalidValue => "invalid value",