WIP #16543 add directives to add or remove trailing slash

Added two directives as said in #16543.

Need to discuss:
- The location of the directives: `PathDirectives` or `RouteDirectives`?
- Returning status code to redirect: 301, 302 or make it configurable? 
- More tests
- More documentation
- etc.
It was to reduce recognition overhead because `SingleSlash` is already used for a pattern matching in a few lines below (`def /` cannot be the LHS of a pattern matching) and to avoid unnecessary redirection, but `/` looks intuitive enough and the latter is obviously a premature optimisation. Will change to `/`.