fix: correct local variable storage and remove premature local slot allocation in vm

The compiler's storeVariable function was incorrectly emitting CODE_STORE_LOCAL for all variables, including locals that already have their value in the correct stack slot. This change renames storeVariable to defineVariable and adds logic to only emit CODE_STORE_GLOBAL for globals, while for locals it emits CODE_DUP to preserve the value through the subsequent POP. Additionally, the vm's callFunction no longer pre-allocates empty slots for locals, as they are now placed on the stack during compilation. New test files verify duplicate local/parameter detection and correct local variable ordering in block scopes.

test/use_true_as_var.wren

@@ -0,0 +1 @@
+var true = "value" // expect error