Add user-defined functions with call frames

Implement fn/return across the full pipeline:
- Lexer: TOK_FN, TOK_RETURN keywords
- Parser: NODE_FN_DEF, NODE_RETURN AST nodes
- Compiler: FunctionEntry table, inline compilation with jump-over and backpatching
- VM: CallFrame stack with variable snapshot for scoped calls and OP_RETURN

projects/custom_fn.j

@@ -0,0 +1,4 @@
+fn greet(name):
+    println("Hola, " + name)
+
+greet("mundo!")