refactor(interpreter): Step 1 - Setup expressions module structure

- Move expressions.rs to expressions/mod.rs
- Create empty module files: operators.rs, calls.rs, access.rs, builtins.rs
- Add module declarations to mod.rs
- Build passes successfully

Next: Step 2 - Extract operator functions

src/interpreter/expressions/access.rs

@@ -0,0 +1,5 @@
+/*!
+ * Field access operations
+ */
+
+use super::*;