[DSLX:ast] Proc config functions require a tuple literal as the last statement

Proc config functions require a tuple literal as the last statement as the implementation currently exists. However, some procs may have `spawn` statements, for example, as their last statement, with a trailing semicolon.

Instead of requiring there literally be a tuple AST node as the last statement that we have to synthesize for later code to rely on, it'd be better if we e.g. did analysis and kept appropriate metadata after typechecking. TL;DR: AST munging bad.