Inject matched variables into `where` predicate scope

Predicates written in `@term RULES` using the `where` clause could benefit from some syntactic sugar. Rather than having to explicitly index into the given match, the scope could automatically be aware of the variables.

```julia
(abs(x) => x) where {σ -> isvalid(Image(σ[x], Nonnegative))} # current
(abs(x) => x) where {isvalid(Image(x, Nonnegative))} # proposed
```

Start by locating the implementation of @term RULES and its where-clause predicate handling; the issue provides no file or test names. Compare the current σ[x] example with the proposed direct x form, and consider the work complete when matched variables are available in that predicate scope without explicit indexing.

Written by the indexing model from the issue text.