Support template parameter/variable expressions in `convert_dim_expr`

[iangneal]

See circom/tests/arrays/array_dimensions_06.circom and #319.

We need to expand support for computing expressions on template parameters and/or variable expressions and using those as array dimensions. This is a current limitation with LLZK itself, which will be addressed in part of an upcoming project on expanding the expressiveness of template parameters (see: https://veridise.notion.site/LLZK-function-templates-296105edf1db80b5a46de053306f07db?source=copy_link)

Once this is fixed in the upstream LLZK library, we can import the functionality here.