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/duplicate_local.wren

@@ -0,0 +1,6 @@
+class Foo {
+  bar {
+    var a = "value"
+    var a = "other" // expect error
+  }
+}

test/duplicate_parameter.wren

@@ -0,0 +1,6 @@
+class Foo {
+  bar(arg,
+      arg) { // expect error
+    "body"
+  }
+}

test/local_collide_with_parameter.wren

@@ -0,0 +1,5 @@
+class Foo {
+  bar(a) {
+    var a = "oops" // expect error
+  }
+}

test/local_in_middle_of_block.wren

@@ -0,0 +1,14 @@
+class Foo {
+  bar {
+    var a = "a"
+    io.write(a) // expect: a
+    var b = a + " b"
+    io.write(b) // expect: a b
+    var c = a + " c"
+    io.write(c) // expect: a c
+    var d = b + " d"
+    io.write(d) // expect: a b d
+  }
+}
+
+Foo.new.bar