Rename MemoryItem to KclValue (#3378)

As @jon points out, "It's not only used in ProgramMemory. It's also
used as the return value of functions and as what gets substituted in
for the pipe substitution symbol %"

src/wasm-lib/derive-docs/tests/doc_comment_with_code.gen

@@ -82,7 +82,7 @@ fn boxed_my_func(
 ) -> std::pin::Pin<
     Box<
         dyn std::future::Future<
-                Output = anyhow::Result<crate::executor::MemoryItem, crate::errors::KclError>,
+                Output = anyhow::Result<crate::executor::KclValue, crate::errors::KclError>,
             > + Send,
     >,
 > {