convmv: fix spurious "error running command echo" on Windows

Before this change the help for convmv was generated by running the
examples each time rclone started up. Unfortunately this involved
running the echo command which did not work on Windows.

This pre-generates the help into `transform.md` and embeds it. It can
be re-generated with `go generate` which is a better solution.

See: https://forum.rclone.org/t/invoke-of-1-70-0-complains-of-echo-not-found/51618

lib/transform/cmap.go

@@ -14,6 +14,9 @@ var (
 	lock  sync.Mutex
 )
 
+// CharmapChoices is an enum of the character map choices.
+type CharmapChoices = fs.Enum[cmapChoices]
+
 type cmapChoices struct{}
 
 func (cmapChoices) Choices() []string {