KCL: Custom snippet values for kcl-in-kcl (#7163)

In #7156, I allowed KCL to set specific snippet completions for each arg of each function. They're optional -- if you don't set one, it'll fall back to the type-driven defaults.

That PR only worked for KCL stdlib functions defined in Rust. This PR enables the same feature, but for functions defined in KCL.

rust/kcl-lib/src/docs/mod.rs

@@ -1040,8 +1040,8 @@ mod tests {
         };
         let snippet = circle_fn.to_autocomplete_snippet();
         assert_eq!(
-            snippet,
-            r#"circle(center = [${0:3.14}, ${1:3.14}], diameter = ${2:3.14})"#
+            snippet, r#"circle(center = [${0:0}, ${1:0}], diameter = ${2:3.14})"#,
+            "actual = left, expected = right"
         );
     }