Feature request: Add support for trailing path (`/`) operator

Currently leading path operators are [supported](https://just.systems/man/en/expressions-and-substitutions.html?highlight=join#joining-paths):
```console
$ bat Justfile

foo := / "b"

$ just --evaluate foo
/b
```

However many tools treat a trailing slash as a significant value (such as `scp`) meaning
`/b` is interpreted differently than `/b/`,

Path operator should be supported as the last identifier for an assignment:

```console
$ bat Justfile

foo := / "b" /

$ just --evaluate foo
/b/
```

Read the expressions and substitutions documentation linked in the issue, then reproduce the examples with a Justfile and `just --evaluate`. Trace how the leading `/` path operator is parsed and evaluated, and confirm that a trailing operator produces `/b/` while existing leading-path behavior remains unchanged.

Written by the indexing model from the issue text.