fix: prevent local variables from being in scope during their own initializer

test/language/variable/global_in_initializer.wren

@@ -1,2 +1 @@
-var a = a == null
-IO.print(a) // expect: true
+var a = a == null // expect error

test/language/variable/local_in_initializer.wren

@@ -1,4 +1,3 @@
-// skip: Variables should not be in scope in their initializer.
 {
   var a = a + 1 // expect error
   IO.print(a)

test/language/variable/shadow_in_initializer.wren

@@ -1,4 +1,3 @@
-// skip: Variables should not be in scope in their initializer.
 {
   var a = "outer"
   {