runtime: move timeUnit to a single place

The timeUnit is now the same type everywhere. Move it to a single place
and add some documentation to it.

src/runtime/runtime_wasip2.go

@@ -12,8 +12,6 @@ import (
 	"internal/cm"
 )
 
-type timeUnit int64
-
 func init() {
 	wasiclirun.Exports.Run = func() cm.BoolResult {
 		callMain()