[RFC] Inspection: function in `where` without type signature

Having functions in `where` blocks without type signatures makes code harder to understand and can lead to subtle compilation errors without a clear understanding of how to fix them. Should implement inspection for this?

Start with the RFC question in this issue and review Stan’s existing inspection conventions before defining the scope. Establish how functions in where blocks should be identified, what diagnostic should be reported, and which cases count as complete; add coverage for the chosen behavior.

Written by the indexing model from the issue text.